Unfortunately, I have to come back to this topic again. As stated above, I was able to deploy a Nextcloud app with default credentials. Also the login sucveeds in that case.
When I tried to change the credentials, the odd behavior appeared again. Also dropping all configuration and datasets and finally setting everything up again with custom credentials does not work.
As mentioned in the other thread (Nextcloud does not start and cannot reinstall after Electric Eel update - #43 by RRD1), I nailed it down to the Redis password. So custom passwords for Nextcloud and Postgres are fine but Redis has to be set to “nextcloud” always.
Does anybody know how to properly set a custom password for Redis here?