DAV and App is doing fine, but the webinterface empty

So the first thing: I dont know how this happened. I’ve set up logos and a color, installed Apps and changed the cron type. After I logged back in, I found a blank page (here a screenshot:)

the strange thing is that DAV is perfectly fine, the mobile App too.
The next thing I did was to revert as many steps as I could (As you see in my screenshot: all HTTP responses are 200, no errors in the nextcloud or apache2 logs). I deactivated plugins, removed the Designchanges and changed the crontype back to AJAX - nothing worked.
What i found out yet:

  1. the URL changed:
    before: https://example.com/index.php/apps/files/?dir=/&fileid=7
    now: https://example.com/index.php/apps/files/?dir=/
  2. opening last used files (which are still displayed) is not working
  3. opening new projects with files will result in a nver stopping loading file manager

I hope i gave you enough infos to reproduce and/or help me. If you need additional infos feel free to comment.