Media Player not working after server update

[seventhwhiskey]

For anyone still running into this: I just had the exact issue as OP after upgrading to 10.9.11 running a Flatpak instance. Deleting the Device from the admin dashboard and also deleting 'jellyfinmediaplayer' and 'Jellyfin Media Player' folders was not effective. Deleting the entire Flatpack folder (~/.var/app/com.github.iwalton3.jellyfin-media-player/) and reinstalling solved the problem.