OK...now I have it:
Only if the X-Box controller is set to P1 in the system, the games start....regal which USB encoders are then for P2 and P3 at the start...STRANGE !!! X-Input and D-Input issue ?
I can't tell you why the controllers behave like that either. I did some research on this but didn't find anything really useful. I'm thinking about switching to a different USB encoder anyway (2 player board). But for now I'm happy and it works...both on the RPI4 and on my new Mini PC. As I said, thank you very much for your great help 👍 😊 👌 !