Had this issue ages ago, then my dad did too a year later on a different client version. Manually changing the “preferred media player” option fixed it on my firestick 4k, 4k Max, and my dads standard firestick.

Jellyfin app>settings menu>Playback>Video section>preferred media player>libVLC (in my case, Exoplayer seemed to be causing the crashes approx 2 years ago but you can try both, I just tried exoplayer again and it doesn’t seem to be crashing either when set manually now so it may have been patched)