I could identify two problems:
1.) rsync - chown - access rights on volume
[1:nextcloud#:nextcloud-app] rsync: [receiver] chown "/var/www/html/3rdparty/...." failed: Operation not permitted (1)
ps -ef shows:
nextclo+ 101005 100993 0 07:15 ? 00:00:01 rsync -rlDog --chown www-data:www-data --delete --exclude-from=/upgrade.exclude /usr/src/nextcloud/ /var/www/html/
so I tried in
[root@ns8 ~] rsync -rlDog --chown 1234:1234 testfolder /mnt/NextCloud/
=> testfolder belongs to 1234
then in
runagent -m nextcloud6
[nextcloud#@ns8 state]$ rsync -rlDog --chown 1234:1234 testfolder /mnt/NextCloud/
=> testfolder belongs to owner of parent folder (nextcloud#)
2.) config of additional containers that are startet by nextcloud#
[1:nextcloud#:podman] Error: lsetxattr /home/nextcloud#/.local/share/containers/storage/volumes/nextcloud-app-data/_data/.htaccess: operation not supported
podman is not aware of the --opt=device=/mnt/NextCloud/ --opt=type=bind nextcloud-app-data change