Anet A8 display on MKS Gen L board
Ok guys I got it! It really was just what is the answer in the other question (Connecting (Anet A8) 2004 display to MKS GEN L)!

My problem was: I used jumper cables and some duct tape to hold them together ... and the only issue was, that I didn't insert it enough. I got mad today and just pushed them really hard more into the AUX2 connector and it suddenly works!