Upgrading Airflow 2.11.2 -> 3.0.6: "Flask app is not initialized" when logging out

[will-m-buchanan]

I'm upgrading from Airflow 2.11.2 to 3.0.6. Our Airflow instance is deployed in kubernetes using custom helm charts, and authenticates through Okta. We use webserver_config.py to override the FabAirflowSecurityManagerOverride class with a custom security manager. All imports and settings in this file have been updated to comply with changes made between v2 and v3 (as far as I know). And in the configs [core] auth manager is set to "airflow.providers.fab.auth_manager.fab_auth_manager.FabAuthManager"

After updating our Okta instance to register the new login redirect URI (v2 had https://<airflow-url>/oauth-authorized/okta, while v3 has https://<airflow-url>/auth/oauth-authorized/okta) I am able to login, but I am still unable to logout.

Clicking the logout button in the UI, I just get a raw JSON response:

{"detail":"Flask app is not initialized. Check that FabAuthManager started up correctly."}

And the api-server logs show:

INFO:     <IP-a>:38782 - "GET /<airflow-path>/api/v2/monitor/health HTTP/1.1" 200 OK
INFO:     <IP-b>:32830 - "GET /<airflow-path>/api/v2/auth/logout HTTP/1.1" 307 Temporary Redirect
INFO:     <IP-b>:32830 - "GET /<airflow-path>/auth/logout HTTP/1.1" 500 Internal Server Error
INFO:     <IP-a>:50668 - "GET /<airflow-path>/api/v2/monitor/health HTTP/1.1" 200 OK

Is it clear from this what is wrong with my settings? How do I check that FabAuthManager started up correctly?