TL;DR: All relevant directories need write permissions for both the new user “nextcloud” and the user “http”.
So I installed Manjaro from scratch, configured a LAMP server, which is working, installed the Nextcloud package and …
Internal Server Error
but at least the php memory errors didn’t show up anymore.
However just by pure coincidence I found out about a setup which works. The introduction of the new “nextcloud” user has left a kind of weird situation where both the http user and the nextcloud user need to have write access to the relevant directories.
E.g. the config directories permissions are correct when user=nextcloud group=http and permissions are 775. The config.php on the other hand works with 660. Here’s the catch: Nextcloud overwrites the file’s permissions back to 640 when maintenance mode is turned on for example. The other directories apps, and data also need wirte permissions both for nextcloud and http user.
Thanks a lot @didi44 for your really kind words and the support!