I finally got it working reliably by setting up a systemd timer executing a script at boot and, just to make sure, every 5 minutes afterwards, that does a chmod -R http:nextcloud and a chown -fR 770 on:
/var/lib/nextcloud
/etc/webapps/nextcloud/config
/usr/share/webapps/nextcloud
Maybe the 770 is overkill for some of the files, but I am in no mood to finetune any longer.
The message complaining about Imagick beeing compiled against a different version (happens to me whenever I use occ), can be suppressed by adding imagick.skip_version_check = 1 to php.ini, if one feels the need to do so. I didn’t.
The Warning no app in context "Invalid data provided to provideInitialState by files" everytime a user opens the files app can be overcome by going into the files app and clicking the + icon then creating a new file and selecting “Set up Templates Folder”. Has to be done for every user. Again, if one feels the need to do so. I didn’t, because I don’t need or want that folder there.
There are still more error messages in the log (I guess because of php8), but I hope they will be ironed out within the next releases of Nextcloud.
All in all upgrading Nextloud 20 to 21 in combination with php8 and the new user nexctloud was just a horrible experience for a pure hobbyist, like myself. Should have listened to my own advice I gave in another thread and should have used docker…