Hi Roelli,
Nice diagram.
As Thorsten suggests, I think you have something swapped. I compared your diagram to my MF_NG wiring: the motor wiring is identical, but I have swapped 5V and GND on the servo track ends compared to your diagram (i.e. I have slider pin 1 going to 5V and pin 3 to GND). I suspect that I have my faders mounted upside-down to convention here :)
So swapping your motor connections around should do the trick. Good luck!
Cheers,
Simon.