I finally got around to take the BMW apart and found out something really weird.
After dismantling the center console, unplugging the AC control unit and taking out the screws from the Heater-Box cover, not only my heat started working normally, also my RPM Gauge works again what failed about 5 month ago. I started to put everything back together and noticed that as soon i plugged back in the AC Controller my heat stopped working on the passenger side again. Just unplugging it again wouldn't do anything until i shut of the car and restarted the engine. It seems that somehow the AC Controller messes up things with the heat.
Does anybody have an explanation for that?