Playback Error

[TheDreadPirate]

Will reinstalling Jellyfin solve this issue? I don't know how to check if "something didn't happen to the DSM Nvidia runtime, that the GPU is still visible to DSM, and that the GPU and Nvidia runtime are being passed into the container".
I am assuming the GPU is still visible to the DSM because I can access the DSM desktop on my remote computer.

Reinstalling Jellyfin probably won't make a difference.  Since it would be pulling in the same configs.  Unless you blow those away and completely start from scratch.  But you should try other things first before going that extreme.

IMO, the problem you are experiencing is due to something going on with DSM since transcoding worked before and nothing changed on the Jellyfin side of things.  Restart the jellyfin container, restart the whole host system.  If that doesn't work,  it is probably better to ask on a Synology focused forum.  Since it is likely the problem originates on the DSM side of things.

Unless a DSM guru wants to chime in here.