It’s basically a duplicate of this one Bug: Using international characters in file names break extra support I just didn’t want to bring up an old post.
So I’m running server version 1.13.8.5395 on Ubuntu 18.4.1 as an LXC on proxmox, but the problem persists as long as I use Plex which is nearly two years.
Now to the problem. I have a separate directory for each movie and their extras in subfolders as it’s described in the plex support articles. When a file of folder in the movie’s directory contains a special character, the extras aren’t shown in the web interface or the app. It doesn’t matter where the special character is, all extras disappear. The movie itself works though.
Here are some characters I’ve tested:
Issue: äöüßàé
works: #"$*!
There is no problem in emby btw.