Hm, I changed this setting to 'clear list and play selected tracks only' or: all tracks.
Now I can again hear what I want to hear.
But, still strange:
I had the 'queue selected tracks next and play now' since I started using MB (as far as I remember).
In the past it did what it says: it inserts the selected track(s) below the actual playing track, then starts playing the inserted track(s) till all done, then continues playing the existing now playing list.
Why is this different now?