Playback modus not working

Hello Team Musescore,
made 3.x update this morning for working with a new partitur. Playback-bar ist grey. No sound if i go with the mouse on a note. Made midi-copy and reloding in 2.x version --> playback and note sound is working.
Before this i checked instrument: piano is activated and restartetd musccore 3.x with factory settings.
Could you help how to run the new version with "sound"?
Regards
Norbert Schwarzer