Sorry for not replying for a long time, I was in contact with iOptron support. But I found the solution for the problem here in the forum:
indilib.org/forum/mounts/8815-ioptron-ge...-issue.html?start=36 
The problem is that the USB on the mount draws too much current. I have now passed the USB cable from the iOptron GEM45 through my Omegon Pro camera and then to the Raspi and now it works. 
Thanks for your help

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