Well, I've checked all kinds of stuff Mike, different device (poly/mk2/ex800) settings, global parameter resets,... It started out with the 'very rare' boot rom mismatch, ended with taking out the hawk, checking flatcables, refluxing all pads but now I'm done with it.
The pot is just working, it shifts the voltage to the correct level in the multiplexer so -for now- I'm happy.