I noticed i forgot to say, that the only fan that is plugged in is the "System Fan 3", so maybe I was just too naive to think that the fan control could still work without most of the fans. However, I never had heating issues, so it would be nice to be able to have fan control that depends on thermal measurements. With this single fan spinning the whole time at 100% the temperature of the CPUs was always below 80 degrees Celsius even at full processor load. The reason why I do not use the other system fans ("System Fan 4" and "System Fan 5") is that they were just too loud (with all 3 system fans plugged in the fan control did not work, too). I don't have CPU fans because I use passive coolers, so cpu fans wouldn't fit.
Anyways it would be nice to be able to control the fan speed of the system fans even without having CPU fans.
I am sorry if I wasted your time by not giving you all information I had.