Okey I got it: its the material of the bed! I dont know why and I dont really care. I took a piece of a 400x400 mm woodplate and started the whole AutobedCompensation process. Now Z0 is truely Z0 everywhere. Crazy accurate. I also moved the probe closer to the nozzle. Now the offset is only 15mm in the y-axis.
Lessons learned: the probe is great(when you use the right bed material)