Hello Erwin,
There is a gasket between the back of the case and the edge of the case. This has to be compressed to relieve the pressure on the little tabs on the back of the case. I used a gluing clamp. I used a set of small srcewdrivers and pushed each of the tabs in on the back of the housing. With each screwdriver disengaging the tabs, I released the clamp and I was able to pry the back off. Once you are inside, be careful, because the pieces are held in place by the back of the case. It sounds like one of the gears is stripped in your motor assembly. They are plastic, so it would not surprise me. Good luck.