[Resolved][problem w/ configuration, not nextcloud] No login possible after (fresh) Nextcloud 15 installation

Nextcloud version: 15.0.0
Operating system and version: OpenSUSE Leap 15.0
Apache or nginx version: nginx 1.14.0
PHP version: 7.2.5

The issue you are facing:
I created a new nextcloud installation with the (web)installer.
In the process I created an administrator user administrator:password, but I could not log in with those credentials after the installer finished.

Trying to login results in a redirection to the file browser and an immediate redirection back to the login; I don’t get to see the file browser itself, only the request in my browser console.

Trying to reset the password results in a CSRF error response from the endpoint is ({"message":"CSRF check failed"}).

(Yes, the browser accepts cookies from this host and correctly provides them to the server)

It also did not work with a backed up instance I moved from another server, which I tried in the first place.

The occ tool seems to work fine, occ check does not report anything.

Is this the first time you’ve seen this error?:
Yes, I guess it might be a problem with my new setup.

So yeah, I have no idea, what to do now.
Is there an efficient way to debug Nextcloud on my server, possibly stepping through the instructions?
Are there any php extensions or settings that could cause such a failure (that is quasi-silent)?

Thanks for your help and merry christmas!

I have now resolved that problem:
As it turns out, the /var/lib/php7 directory, where PHP sessions are kept, was owned by wwwrun:root with a mod of 755 .
But since the PHP pool was running as nginx:nobody , it could not create sessions there.

So in the end it was a permissions problem.
Unfortunately, this error was reported nowhere, not even the php-fpm error log.
In the end I found it using Xdebug.