How to debug oauth2 properly?

I’m trying to debug whats happening to the oauth2 authentication. After logging in, i just get back to the login page. No errors, no error logs.

I already found the DEBUG and CMD_LOGLEVEL environment variable (docker-compose setup, 1.9.6). But i’m still lost. Interesting is that the auth provider (authentik, setup as described there) does not seem to get a request from hedgedoc - but there are no real logs on hedgedoc side describing what’s going on. Or at least i did not find them.

Is there a way to see whats going on?