5 hours ago
OK - I've solved at least part of this. It was - as you first suggested - a "Permissions" issue, but not a straightforward one at all. The answer was not in the Jellyfin configuration, or in the metadata... The answer was in an arcane configuration area of Synology; in particular the Permissions for something called "System Internal Users". AFAIK, this is unique to DSM (the Synology "OS").
The (partial) solution was to set permissions for "System Internal Users" for "movtvshows" share to be the same as those for the "movies" share. The "movies" share existed when Jellyfin was installed, but "movtvshows" did not. This was the key to recognizing what the fault was likely to be.
It's still very disappointing that none of this was mentioned in the documentation... I wasted a lot of my time, and more than a little of yours. I'll try to mark this as "SOLVED", and look for an opportunity to add this to the documentation. And thanks again for your help!
The (partial) solution was to set permissions for "System Internal Users" for "movtvshows" share to be the same as those for the "movies" share. The "movies" share existed when Jellyfin was installed, but "movtvshows" did not. This was the key to recognizing what the fault was likely to be.
It's still very disappointing that none of this was mentioned in the documentation... I wasted a lot of my time, and more than a little of yours. I'll try to mark this as "SOLVED", and look for an opportunity to add this to the documentation. And thanks again for your help!