I've had trouble getting Jellyfish to stream anything. The error message it gives you in the client GUI make it sound like the problem is client-side, but it always ends up being permissions on the server (for me, anyway).
I ended up having to move my media drive into /mnt/ (rather than /mnt//) and allow Jellyfin to access that folder - the Jellyfin service needs permissions.
The Jellyfin log files will show you that is having trouble accessing the files of this is the issue.
The Voyager app wouldn't let me edit my comment for some reason... So here's the edit I was going to add.
I really dislike AI but the most concise set of instructions I could find was googling "Linux Mint drive permissions jellyfin" and following the instructions the AI spits out. Definitely read through the instructions and make sure they seem same first, tho.