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.