Starting up
The web interface is ready but the backend service is not up yet. On a first start or right after an upgrade this usually takes a few seconds; this page refreshes itself and moves on once the backend answers.
Waiting for the backend
If it stays here: check that the container is still running and look at its logs for errors. No sign-in form is shown until the system has finished initializing — signing in would fail anyway.
Waiting for the backend…