Hi SirTux, thank you for having a look and your reply!
As advised, I just tried to directly mount the data directory but it is still not working (this is what I get in my Browser-Window now):
"App directory "/var/www/owncloud/custom" not found! Please put the ownCloud apps folder in the ownCloud folder or the folder above. You can also configure the location in the config.php file."
Here is what I did:
service apache2 stop
cd /var/lib/univention-appcenter/apps/owncloud/data
rsync -avz /var/lib/univention-appcenter/apps/owncloud/data /mnt/owncloud
rm -r *
umount -a
vi /etc/fstab (as follows:)
//windows-server/owncloud/data /var/lib/univention-appcenter/apps/owncloud/data cifs username=USER,password=PASSWORD,domain=MYDOMAIN,uid=www-data 0 0service
mount -a
service apache2 start
There is still something wrong or missing …
When I restart ownCloud (univention-app restart owncloud), I get this now:
ERROR: for owncloud_redis_1 b'Cannot restart container da5c740e9fe346fd4fd559cb5ae4498c4bc362a737cd7344555be07095fdea6d: OCI runtime create failed: container_linux.go:345: starting container process caused "process_linux.go:281: applying cgroup configuration for process caused \\"mountpoint for devices not found\\"": unknown'