Kempermaniac: Forgot to mention, but of course I tried this as well. I even tried all the USB slots with every single of the four USB cables. I don't use a USB-hub. Just the slots of the mainboard itself.
Sixstringedmatt: I agree with you that it is rather a problem with the Mainboard, maybe it is not grounded properly?
I did not try the two front USB slot, I got to admit. But since my KPA should always be connected to the PC I don't wanna have tons of cables hanging around in front of the PC....
I think I will try a different PC. Thx so far for the help!