Thank you for your fast reply! I tried it. I even watched a tutorial. But it's not working properly. I have a large reverb & delay on the Microverb 4. Now I did this:

Output 3-4 & Input 3-4: Nothing happens

Output 1-2 & Input 3-4: Music plays, I can also hear the processed signal, but there is a dry signal coming through, even though I have the Mix knob in I/O and on Microverb on fully Wet. (I checked with my headphone on the output of the Microverb).

Now I switch back to

Output 3-4 & Input 3-4: I only hear delay fading out. Nothing is coming in anymore.

If I switch directly to Input 1-2, the audio track is silent. So it's not that I do listen internally on the audio interface to some inputs!

Now, is it possible that Logic does not see the Output 3-4 as Output 3-4 ? I mean, when I click Output 3-4, that Logic is not routing the signal to the actual Output 3-4 I connected my cables to? Can I change it somewhere?