diff --git a/nextcloud_mcp_server/api/passwords.py b/nextcloud_mcp_server/api/passwords.py
index 19ae1bdf..6f29892f 100644
--- a/nextcloud_mcp_server/api/passwords.py
+++ b/nextcloud_mcp_server/api/passwords.py
@@ -241,7 +241,7 @@ async def provision_app_password(request: Request) -> JSONResponse:
     # Parse optional scopes and the Nextcloud loginName from the request body
     # up front. Nextcloud authenticates app passwords against the *loginName*,
     # which can differ from the UID — e.g. OIDC-provisioned users whose UID is
-    # their display name (UID "Chris Coutinho", loginName "chris@coutinho.io").
+    # their display name (UID "Ada Lovelace", loginName "ada@example.com").
     # Use the loginName for the BasicAuth validation below, falling back to the
     # path user_id for legacy callers that don't send one (where UID ==
     # loginName).
diff --git a/nextcloud_mcp_server/auth/storage.py b/nextcloud_mcp_server/auth/storage.py
index 4d58069d..e6b95bac 100644
--- a/nextcloud_mcp_server/auth/storage.py
+++ b/nextcloud_mcp_server/auth/storage.py
@@ -2130,14 +2130,22 @@ class RefreshTokenStorage:
         removed: list[str] = []
 
         async def _validate_user(user_id: str) -> None:
-            app_password = await self.get_app_password(user_id)
-            if not app_password:
-                return
-
             try:
+                app_data = await self.get_app_password_with_scopes(user_id)
+                if not app_data:
+                    return
+
+                app_password = app_data["app_password"]
+                # Authenticate as the stored loginName, not the UID: Nextcloud
+                # keys app-password auth on the loginName, which differs from
+                # the UID for OIDC-provisioned users. Using the UID here would
+                # 401 a *valid* password and wrongly delete it. Falls back to
+                # the UID for legacy rows without a stored loginName.
+                login_name = app_data.get("username") or user_id
+
                 async with httpx.AsyncClient(
                     base_url=nextcloud_host,
-                    auth=httpx.BasicAuth(user_id, app_password),
+                    auth=httpx.BasicAuth(login_name, app_password),
                     timeout=10.0,
                 ) as client:
                     response = await client.get(
diff --git a/nextcloud_mcp_server/client/__init__.py b/nextcloud_mcp_server/client/__init__.py
index ce41f47d..e76d324e 100644
--- a/nextcloud_mcp_server/client/__init__.py
+++ b/nextcloud_mcp_server/client/__init__.py
@@ -102,10 +102,21 @@ class NextcloudClient:
         username: str,
         auth: Auth | None = None,
         *,
+        auth_username: str | None = None,
         password: str | None = None,
         token: str | None = None,
     ):
+        # ``username`` is the Nextcloud UID — it drives DAV/API path
+        # construction (e.g. ``/remote.php/dav/files/<uid>/``). ``auth_username``
+        # is the credential identity Nextcloud authenticates the app password
+        # against (the loginName), which differs from the UID for
+        # OIDC-provisioned users. Defaults to ``username`` so single-user and
+        # OAuth modes (where UID == loginName) are unchanged. Callers pass the
+        # matching ``auth=BasicAuth(auth_username, ...)`` for the httpx leg;
+        # ``auth_username`` is threaded to the CalDAV client, which builds its
+        # own auth object from the raw credential.
         self.username = username
+        auth_username = auth_username or username
         self._client = AsyncClient(
             base_url=base_url,
             auth=auth,
@@ -122,7 +133,11 @@ class NextcloudClient:
         # its preferred backend in v3.x) builds a backend-compatible auth object
         # itself — passing httpx.BasicAuth here breaks under niquests (#731).
         self.calendar = CalendarClient(
-            base_url, username, password=password, token=token
+            base_url,
+            username,
+            auth_username=auth_username,
+            password=password,
+            token=token,
         )
         self.contacts = ContactsClient(self._client, username)
         self.cookbook = CookbookClient(self._client, username)
diff --git a/nextcloud_mcp_server/client/calendar.py b/nextcloud_mcp_server/client/calendar.py
index 54723646..11c3f1c1 100644
--- a/nextcloud_mcp_server/client/calendar.py
+++ b/nextcloud_mcp_server/client/calendar.py
@@ -42,6 +42,7 @@ class CalendarClient:
         base_url: str,
         username: str,
         *,
+        auth_username: str | None = None,
         password: str | None = None,
         token: str | None = None,
     ):
@@ -55,7 +56,10 @@ class CalendarClient:
 
         Args:
             base_url: Nextcloud base URL
-            username: Nextcloud username
+            username: Nextcloud username (UID) — used for DAV path construction
+            auth_username: Credential identity (loginName) the app password
+                authenticates against; defaults to ``username``. Differs from
+                the UID for OIDC-provisioned users.
             password: App password / login password — selects ``auth_type="basic"``
             token: OAuth bearer token — selects ``auth_type="bearer"``
 
@@ -64,6 +68,11 @@ class CalendarClient:
         """
         self.username = username
         self.base_url = base_url
+        # The UID (``username``) drives DAV path construction; the loginName
+        # (``auth_username``) is the credential the app password authenticates
+        # against. They differ for OIDC-provisioned users. Defaults to the UID
+        # so existing single-user / OAuth callers are unchanged.
+        auth_username = auth_username or username
 
         auth_kwargs: dict[str, Any] = {}
         if password is not None:
@@ -74,7 +83,7 @@ class CalendarClient:
         # AsyncDAVClient needs the full base URL for proper URL construction
         self._dav_client = AsyncDAVClient(
             url=f"{base_url}/remote.php/dav/",
-            username=username,
+            username=auth_username,
             ssl_verify_cert=get_nextcloud_ssl_verify(),  # type: ignore[arg-type]  # caldav types say bool|str but passes through to niquests which accepts SSLContext
             **auth_kwargs,
         )
diff --git a/nextcloud_mcp_server/context.py b/nextcloud_mcp_server/context.py
index d4b4cae3..2d8d0ec4 100644
--- a/nextcloud_mcp_server/context.py
+++ b/nextcloud_mcp_server/context.py
@@ -191,13 +191,27 @@ async def _get_client_from_login_flow(
             "Call nc_auth_provision_access to complete Login Flow."
         )
 
-    username = app_data.get("username") or user_id
+    # Authenticate with the stored Nextcloud loginName, but build DAV/API paths
+    # with ``user_id`` — the identity the rest of the system is keyed on (the
+    # app-password store, vector payloads, scopes, and the background-sync
+    # ``get_user_client_basic_auth`` all use it). For OIDC-provisioned users the
+    # loginName (e.g. an email) differs from this identity, and Nextcloud
+    # authenticates app passwords against the loginName. Falls back to
+    # ``user_id`` for legacy rows stored without a loginName.
+    login_name = app_data.get("username") or user_id
+    app_password = app_data["app_password"]
 
-    logger.debug("Creating Login Flow v2 client for %s as %s", nextcloud_host, username)
+    logger.debug(
+        "Creating Login Flow v2 client for %s (id=%s, login=%s)",
+        nextcloud_host,
+        user_id,
+        login_name,
+    )
 
     return NextcloudClient(
         base_url=nextcloud_host,
-        username=username,
-        auth=BasicAuth(username, app_data["app_password"]),
-        password=app_data["app_password"],
+        username=user_id,
+        auth_username=login_name,
+        auth=BasicAuth(login_name, app_password),
+        password=app_password,
     )
diff --git a/nextcloud_mcp_server/vector/oauth_sync.py b/nextcloud_mcp_server/vector/oauth_sync.py
index ad37556d..fe6f7a4f 100644
--- a/nextcloud_mcp_server/vector/oauth_sync.py
+++ b/nextcloud_mcp_server/vector/oauth_sync.py
@@ -114,20 +114,28 @@ async def get_user_client_basic_auth(
     if storage is None:
         storage = await _get_initialized_basic_auth_storage()
 
-    # Retrieve app password from local storage
-    app_password = await storage.get_app_password(user_id)
+    # Retrieve app password (and the stored Nextcloud loginName) from local
+    # storage. Nextcloud authenticates app passwords against the loginName,
+    # which differs from the UID for OIDC-provisioned users; authenticate as
+    # the loginName while keeping the UID for DAV/API path construction. Falls
+    # back to the UID for legacy rows stored without a loginName.
+    app_data = await storage.get_app_password_with_scopes(user_id)
 
-    if not app_password:
+    if not app_data:
         raise NotProvisionedError(
             f"User {user_id} has not provisioned an app password. "
             f"User must configure background sync in Astrolabe personal settings."
         )
 
+    app_password = app_data["app_password"]
+    login_name = app_data.get("username") or user_id
+
     logger.info("Using app password for background sync: %s", user_id)
     return NextcloudClient(
         base_url=nextcloud_host,
         username=user_id,
-        auth=BasicAuth(user_id, app_password),
+        auth_username=login_name,
+        auth=BasicAuth(login_name, app_password),
         password=app_password,
     )
 
diff --git a/tests/unit/client/test_calendar.py b/tests/unit/client/test_calendar.py
index a2e0e92a..bb7b15ce 100644
--- a/tests/unit/client/test_calendar.py
+++ b/tests/unit/client/test_calendar.py
@@ -97,3 +97,48 @@ def test_password_takes_precedence_over_token(mocker):
     call_kwargs = mock_dav_client.call_args.kwargs
     assert call_kwargs["password"] == "app-pw"
     assert call_kwargs["auth_type"] == "basic"
+
+
+def test_auth_username_used_for_credential_uid_for_path(mocker):
+    """OIDC users: the loginName authenticates, the UID builds the DAV path.
+
+    Nextcloud keys app-password auth on the loginName (which can differ from
+    the UID), but ``/remote.php/dav/calendars/<uid>/`` must use the UID. The
+    two must not be conflated.
+    """
+    mock_dav_client = mocker.patch(
+        "nextcloud_mcp_server.client.calendar.AsyncDAVClient"
+    )
+
+    from nextcloud_mcp_server.client.calendar import CalendarClient
+
+    client = CalendarClient(
+        "https://cloud.example.org",
+        "Ada Lovelace",  # UID
+        auth_username="ada@example.com",  # loginName
+        password="app-pw-1234",
+    )
+
+    # Credential identity → loginName
+    assert mock_dav_client.call_args.kwargs["username"] == "ada@example.com"
+    # Path identity → UID
+    assert client.username == "Ada Lovelace"
+    assert (
+        client._calendar_home_url
+        == "https://cloud.example.org/remote.php/dav/calendars/Ada Lovelace/"
+    )
+
+
+def test_auth_username_defaults_to_username(mocker):
+    """Backwards compat: without ``auth_username`` the UID is used for both,
+    so single-user / OAuth callers (UID == loginName) are unchanged.
+    """
+    mock_dav_client = mocker.patch(
+        "nextcloud_mcp_server.client.calendar.AsyncDAVClient"
+    )
+
+    from nextcloud_mcp_server.client.calendar import CalendarClient
+
+    CalendarClient("https://cloud.example.org", "alice", password="app-pw")
+
+    assert mock_dav_client.call_args.kwargs["username"] == "alice"
diff --git a/tests/unit/test_app_password_loginname_auth.py b/tests/unit/test_app_password_loginname_auth.py
new file mode 100644
index 00000000..0f57bb4b
--- /dev/null
+++ b/tests/unit/test_app_password_loginname_auth.py
@@ -0,0 +1,216 @@
+"""Unit tests: stored-app-password client builders authenticate with the
+Nextcloud loginName, not the UID.
+
+Regression for the OIDC-provisioned-user case where the Nextcloud UID differs
+from the loginName (e.g. UID "Ada Lovelace", loginName "ada@example.com").
+Nextcloud authenticates app passwords against the loginName, so binding the UID
+as the BasicAuth username returns HTTP 401 for every Notes/Files/Shares/CalDAV
+call — which previously stopped the background sync scan loop and degraded
+ACL-aware search to a self-only owner filter.
+
+The builders must split the two identities:
+- credential username (httpx + CalDAV auth) → loginName
+- DAV/URL path identity (``client.username``)  → UID
+"""
+
+import tempfile
+from pathlib import Path
+
+import httpx
+import pytest
+from cryptography.fernet import Fernet
+
+from nextcloud_mcp_server.auth.storage import RefreshTokenStorage
+
+pytestmark = pytest.mark.unit
+
+_APP_PW = "aaaaa-bbbbb-ccccc-ddddd-eeeee"
+
+
+def _basic_auth_header(username: str, password: str) -> str:
+    return httpx.BasicAuth(username, password)._auth_header
+
+
+@pytest.fixture
+async def temp_storage():
+    """Encrypted storage backed by a throwaway SQLite file."""
+    with tempfile.TemporaryDirectory() as tmpdir:
+        storage = RefreshTokenStorage(
+            db_path=str(Path(tmpdir) / "test.db"),
+            encryption_key=Fernet.generate_key().decode(),
+        )
+        await storage.initialize()
+        yield storage
+
+
+class _FakeAsyncClient:
+    """Minimal stand-in for ``httpx.AsyncClient`` that records the ``auth=``
+    kwarg and returns a canned status code."""
+
+    def __init__(self, status_code: int):
+        self._status_code = status_code
+        self.captured_auth: httpx.Auth | None = None
+
+    def __call__(self, *args, **kwargs):
+        self.captured_auth = kwargs.get("auth")
+        return self
+
+    async def __aenter__(self):
+        return self
+
+    async def __aexit__(self, *exc):
+        return False
+
+    async def get(self, *args, **kwargs):
+        return httpx.Response(self._status_code)
+
+
+async def test_basic_auth_builder_splits_uid_and_loginname(mocker):
+    """multi_user_basic background-sync builder: loginName authenticates,
+    UID builds paths."""
+    mock_dav_client = mocker.patch(
+        "nextcloud_mcp_server.client.calendar.AsyncDAVClient"
+    )
+    from nextcloud_mcp_server.vector.oauth_sync import get_user_client_basic_auth
+
+    storage = mocker.MagicMock()
+    storage.get_app_password_with_scopes = mocker.AsyncMock(
+        return_value={
+            "app_password": "app-pw-1234",
+            "username": "ada@example.com",  # loginName
+            "scopes": None,
+        }
+    )
+
+    client = await get_user_client_basic_auth(
+        "Ada Lovelace",  # UID
+        "https://cloud.example.org",
+        storage=storage,
+    )
+
+    # Path identity → UID
+    assert client.username == "Ada Lovelace"
+    # httpx credential (notes/webdav/sharing) → loginName
+    assert client._client.auth._auth_header == _basic_auth_header(
+        "ada@example.com", "app-pw-1234"
+    )
+    # CalDAV credential → loginName, but calendar-home path → UID
+    assert mock_dav_client.call_args.kwargs["username"] == "ada@example.com"
+    assert client.calendar.username == "Ada Lovelace"
+
+
+async def test_basic_auth_builder_legacy_row_falls_back_to_uid(mocker):
+    """Legacy rows stored before the loginName column was populated have
+    ``username = None`` → fall back to the UID for the credential (preserves
+    the previous behaviour where UID == loginName)."""
+    mock_dav_client = mocker.patch(
+        "nextcloud_mcp_server.client.calendar.AsyncDAVClient"
+    )
+    from nextcloud_mcp_server.vector.oauth_sync import get_user_client_basic_auth
+
+    storage = mocker.MagicMock()
+    storage.get_app_password_with_scopes = mocker.AsyncMock(
+        return_value={"app_password": "pw", "username": None, "scopes": None}
+    )
+
+    client = await get_user_client_basic_auth(
+        "alice", "https://cloud.example.org", storage=storage
+    )
+
+    assert client.username == "alice"
+    assert client._client.auth._auth_header == _basic_auth_header("alice", "pw")
+    assert mock_dav_client.call_args.kwargs["username"] == "alice"
+
+
+async def test_basic_auth_builder_unprovisioned_raises(mocker):
+    """No stored app password → NotProvisionedError (unchanged contract)."""
+    from nextcloud_mcp_server.vector.oauth_sync import (
+        NotProvisionedError,
+        get_user_client_basic_auth,
+    )
+
+    storage = mocker.MagicMock()
+    storage.get_app_password_with_scopes = mocker.AsyncMock(return_value=None)
+
+    with pytest.raises(NotProvisionedError):
+        await get_user_client_basic_auth(
+            "alice", "https://cloud.example.org", storage=storage
+        )
+
+
+async def test_login_flow_builder_splits_id_and_loginname(mocker):
+    """Login Flow v2 per-request builder (MCP tool path): same split as the
+    background-sync builder. ``user_id`` (the identity the system is keyed on,
+    == NC UID for NC-as-OIDC-IdP) builds DAV paths; the stored loginName
+    authenticates. Previously the loginName was used for the DAV path too,
+    which is wrong for OIDC users (UID != loginName)."""
+    mock_dav_client = mocker.patch(
+        "nextcloud_mcp_server.client.calendar.AsyncDAVClient"
+    )
+    from nextcloud_mcp_server import context
+
+    mocker.patch(
+        "nextcloud_mcp_server.auth.token_utils.extract_user_id_from_token",
+        mocker.AsyncMock(return_value="Ada Lovelace"),  # token sub == NC UID
+    )
+    storage = mocker.MagicMock()
+    storage.get_app_password_with_scopes = mocker.AsyncMock(
+        return_value={
+            "app_password": "app-pw-9999",
+            "username": "ada@example.com",  # loginName
+            "scopes": None,
+        }
+    )
+    mocker.patch.object(
+        context, "get_shared_storage", mocker.AsyncMock(return_value=storage)
+    )
+
+    client = await context._get_client_from_login_flow(
+        mocker.MagicMock(), "https://cloud.example.org"
+    )
+
+    # Path identity → user_id (== UID); credential → loginName
+    assert client.username == "Ada Lovelace"
+    assert client._client.auth._auth_header == _basic_auth_header(
+        "ada@example.com", "app-pw-9999"
+    )
+    assert mock_dav_client.call_args.kwargs["username"] == "ada@example.com"
+
+
+async def test_cleanup_authenticates_with_loginname_not_uid(temp_storage, mocker):
+    """cleanup_invalid_app_passwords must validate with the stored loginName.
+
+    Validating as the UID would 401 a *valid* OIDC password and wrongly delete
+    it — the exact failure observed on a login_flow tenant in production.
+    """
+    await temp_storage.store_app_password_with_scopes(
+        "Ada Lovelace", _APP_PW, username="ada@example.com"
+    )
+    fake = _FakeAsyncClient(status_code=200)  # valid credential
+    mocker.patch("nextcloud_mcp_server.auth.storage.httpx.AsyncClient", fake)
+
+    removed = await temp_storage.cleanup_invalid_app_passwords(
+        "https://cloud.example.org"
+    )
+
+    assert removed == []  # valid password preserved
+    assert fake.captured_auth._auth_header == _basic_auth_header(
+        "ada@example.com", _APP_PW
+    )
+    assert await temp_storage.get_app_password("Ada Lovelace") == _APP_PW
+
+
+async def test_cleanup_removes_genuinely_invalid_password(temp_storage, mocker):
+    """A real 401 still removes the stored password (unchanged contract)."""
+    await temp_storage.store_app_password_with_scopes(
+        "alice", _APP_PW, username="alice"
+    )
+    fake = _FakeAsyncClient(status_code=401)
+    mocker.patch("nextcloud_mcp_server.auth.storage.httpx.AsyncClient", fake)
+
+    removed = await temp_storage.cleanup_invalid_app_passwords(
+        "https://cloud.example.org"
+    )
+
+    assert removed == ["alice"]
+    assert await temp_storage.get_app_password("alice") is None
diff --git a/tests/unit/test_management_app_password_endpoints.py b/tests/unit/test_management_app_password_endpoints.py
index 0a6fcb55..7b62ac3e 100644
--- a/tests/unit/test_management_app_password_endpoints.py
+++ b/tests/unit/test_management_app_password_endpoints.py
@@ -257,7 +257,7 @@ async def test_provision_app_password_success(temp_storage, mocker):
 async def test_provision_app_password_uses_loginname_not_uid(temp_storage, mocker):
     """Regression: when the Nextcloud UID differs from the loginName (e.g.
     OIDC-provisioned users whose UID is their display name — UID
-    "Chris Coutinho", loginName "chris@coutinho.io"), the OCS BasicAuth
+    "Ada Lovelace", loginName "ada@example.com"), the OCS BasicAuth
     validation must authenticate as the loginName from the request body, not
     the UID. Authenticating as the UID is rejected by Nextcloud with HTTP 401.
     """
@@ -273,7 +273,7 @@ async def test_provision_app_password_uses_loginname_not_uid(temp_storage, mocke
     # OCS validation succeeds and reports the UID as the account id.
     mock_response = MagicMock()
     mock_response.status_code = 200
-    mock_response.json.return_value = {"ocs": {"data": {"id": "Chris Coutinho"}}}
+    mock_response.json.return_value = {"ocs": {"data": {"id": "Ada Lovelace"}}}
 
     mock_client = AsyncMock()
     mock_client.get = AsyncMock(return_value=mock_response)
@@ -291,9 +291,9 @@ async def test_provision_app_password_uses_loginname_not_uid(temp_storage, mocke
     # A literal space in the path is encoded by the client and decoded back to
     # the UID; the BasicAuth username matches that UID.
     response = client.post(
-        "/api/v1/users/Chris Coutinho/app-password",
-        headers={"Authorization": create_basic_auth_header("Chris Coutinho", pw)},
-        json={"username": "chris@coutinho.io"},
+        "/api/v1/users/Ada Lovelace/app-password",
+        headers={"Authorization": create_basic_auth_header("Ada Lovelace", pw)},
+        json={"username": "ada@example.com"},
     )
 
     assert response.status_code == 200
@@ -301,10 +301,10 @@ async def test_provision_app_password_uses_loginname_not_uid(temp_storage, mocke
 
     # The OCS BasicAuth used the loginName from the body, not the UID.
     _, get_kwargs = mock_client.get.call_args
-    assert get_kwargs["auth"] == ("chris@coutinho.io", pw)
+    assert get_kwargs["auth"] == ("ada@example.com", pw)
 
     # Stored under the UID (the identity key).
-    assert await temp_storage.get_app_password("Chris Coutinho") == pw
+    assert await temp_storage.get_app_password("Ada Lovelace") == pw
 
 
 async def test_provision_app_password_nextcloud_validation_fails(mocker):