Your support told me to bypass the mixer to do this:
I route a signal from Studio One to OUT3 of the Presonus Quantum HD8 to send the signal to an external compressor the signal is output according to Studio One. You can see this on OUT3 in Studio One. However, no signal arrives at the compressor and therefore no compressed signal returns to IN3 of the Quantum HD8. With the mixer bypassed in Universal Control it works. But I also want to switch the speakers A and B. And this only works when the mixer ist not bypassed.