I'm having the same issue, I checked the logs in truenas while the app is starting and get the following:
2026-03-22 10:50:51.995861+00:00Unhandled exception. System.UnauthorizedAccessException: Access to the path '/cache/.jellyfin-cache' is denied.
2026-03-22 10:50:51.995892+00:00---> System.IO.IOException: Permission denied
2026-03-22 10:50:51.995900+00:00--- End of inner exception stack trace ---
2026-03-22 10:50:51.995906+00:00at Interop.ThrowExceptionForIoErrno(ErrorInfo errorInfo, String path, Boolean isDirError)
2026-03-22 10:50:51.995916+00:00at Microsoft.Win32.SafeHandles.SafeFileHandle.Open(String path, OpenFlags flags, Int32 mode, Boolean failForSymlink, Boolean& wasSymlink, Func`4 createOpenException)
2026-03-22 10:50:51.995922+00:00at Microsoft.Win32.SafeHandles.SafeFileHandle.Open(String fullPath, FileMode mode, FileAccess access, FileShare share, FileOptions options, Int64 preallocationSize, UnixFileMode openPermissions, Int64& fileLength, UnixFileMode& filePermissions, Boolean failForSymlink, Boolean& wasSymlink, Func`4 createOpenException)
It seems like it cannot access the /cache folder. Will investigate further.