This seems to be a file permission issue

On console can you do

ls -la

Inside the media folder and see the owner user, owner group and permission?

If you want to do quick and dirty test do

chmod 777 <filename>

Then try and play that file.

If that works, try 775 and then 755 to tighten the permissions a bit

Check the logs. There might be suggestion to ffmpeg or decoder configs in jellyfin. The pi probably wont support some of them

Damn

Came across this last night

https://github.com/niedev/RTranslator

Lemmy community is awesome