I figured it out now! The boards were working on their own, but the problem was that when I connected the two together, I mixed up the cables! I thought with AC it is impossible to do a reverse polarity error, but this is only true if you have only a single board 
So I did not connect the two ground planes of my boards together, but connected the ground of one to the .
I am still wondering a little why I did not get a short immediately, but could measure the correct voltages of +12 and -12V on the outputs. Only when I plugged something in, it all went dark…
Anyway, thank you for your help and now I can add a lot more modules!! 
3 Likes