Hi Christian,
I think ACLs are to blame.
What is the exact reaction you got to a specific command?
In my case:
# ls -l /home/ |grep wbk
drwxrwxr-x+ 5 wbk wbk 4096 Jan 5 2023 wbk
# ls -l /home/yunohost.app/nextcloud/data/ |grep wbk
drwxr-x---+ 8 nextcloud nextcloud 4096 Jul 9 2020 wbk
Notice the + behind the permissions. It indicates additional permissions via ACL are active.
# getfacl /home/yunohost.app/nextcloud/data/wbk/
getfacl: Removing leading '/' from absolute path names
# file: home/yunohost.app/nextcloud/data/wbk/
# owner: nextcloud
# group: nextcloud
user::rwx
group::r-x
group:fusers:rwx #effective:r-x
mask::r-x
other::---
default:user::rwx
default:group::r-x
default:group:13362:rwx
default:mask::rwx
default:other::---
I don’t have a direct answer which changes to make to ACL in your case, but I found an old thread where there is a similar situation, Getting Jellyfin and NextCloud on the same page - #13 by tituspijean
tituspijean got a suggestion there with an intermediate group for those files, of which Nextcloud as well as Jellyfin are members.
When changing ACL with setfacl, take care that there is one option to change the current files, and another to change new files added in the future.
Good luck!