Sorry, I guess I misinterpreted this thread’s error message.
I would have been right here:
https://forums.plex.tv/t/android-app-crashes-or-gives-error-when-trying-to-play-synced-music-an-error-occurred-while-playing-this-item/ 612534
Synchronization works, but I cannot play the local files. Play directly from the server works.
As I already wrote, when playing the synchronized MP3 file, the information says it is an MKV container.
Thanks for your help…