Before the reinstall of the 31st I deleted the setup and thought to have also deleted the Jellyfin configuration in the iX-Volume.
I subsequently installed Jellyfin again from scratch, setting /media/tv as the Jellyfin container series path and rescanned the directories.
However I noticed that the series path in the above log file starts with: /tv
It appears that for somehow the old Jellyfin configuration was not deleted, but not clear why I could play the seres from the Jellyfin GUI.
I just deleted Jellyfin again, deleting the configuration, rebooted and verified the iX-Volume deletion.
Now reinstalling Jellyfin, setting the correct Jellyfin container path, rescanning and enabling DLNA all works fine again.
All files are in the “New series” and “New movies” folders now and are playable.