Can't start process thread using umc204 audio interface

Hi there! I’m a n00b starting to like ardour and I now want to record some audio tracks. When I open up Ardour and choose my audio interface (Behringer UMC204HD) in the session dialogue as the input source it sais “couldn’t start process thread” or something similar (loosely translated from german). If I only choose internal input and output it starts as usual but then of course I can’t record using the interface.
I’m using macOS Catalina 10.15.7 and Ardour 6.8.0
Does anyone have a clue what’s the problem?