Ok, update I put in, that didn't help, but ....
... I may have found a cause, though not a solution. Actually I use the Musicbee UPnP app, which worked fine so far. As a player I then use my Naim Unitqute. If I set the player back to "Directsound", the problem no longer occurs.
But I still have no idea how I could change this with the UPnP player, or what could have changed.