first off all ... sorry for the late reply.
As this problem appears with the same fixture (MH Varytec Hero Spot 90) I think this is not the reason in my case.
Sometimes the beam preview is correct and than, creating a new scene in the same showfile, the beam preview is unexpected. I can't see any relation for this. It happens with or without colours, gobos, moving or each other. It seems to be random.
Let there be light ...