Thx SWAT_OP-R8R and SilverFox it indeed was the auto hide option, but I don't remember switching the option on... strange. Anyways switching it off helped and everything works fine now.
Just an update on the behaviour of the auto hide: moving mouse to the top of the screen didn't work, but I figured out that the menu was still there but with 100% transparency. The buttons could still be pressed but couldn't be seen - see the second image.
Anyways, thanks for the quick and helpful response.