I wanted to thank you all for your input.
Atm the main-problem is still not solved but before further hunting for the overextrusion-issue and all the suffering :), i thought it is a good time to replace the old part-cooling-system which i got from Grabcad and never convinced me because the ducts were oriented 90° directly to bottom (but it looked awsome )
So i opened my old loved Rhino3D an started to model a new cooling-system from scratch, simple, but hopefully well performing:
I keep you updated ! When i approved that the new cooling fans are doing well, i will provide the model on grabcad.
Hunting over-extrusion will start soon again.