MM-control board voltage issue causing erratic filament sensor readings
So after struggling with the MMU2S for like an eternity now I finally traced down one of my problems to my MM-control board. My proximity sensor (FINDA) seems to not work well with voltages under 4.5V or so, which I verified using an external supply. Measuring the logic 5V supply voltage of the MM-control board I got 4.3V. That explains a lot...
Culprit was the reverse polarity protection diode D1 (BOMd as SK3109MA-DIO), see https://github.com/prusa3d/MM-control-2.0/blob/master/rev.03/MM-control.pdf. The voltage drop of that schottky diode is just enough to make my proximity sensor work erratically. I removed that diode and replaced it with a jumper and voila, the proximity sensor works reliably now.
Of course this mod means that if I ever plug in the header (P1) in reverse the board could blow up and emit magic smoke. So this is not a recommended remedy unless you know what you are doing.