@Smithy
thanks for the quick response.
I tried different speed settings in Cura, but the result was not the one, which I expected.
So I focused on the hardware: with the XY-test, I found out very fast, that the bolts, which fix the X- and Y-servos to the housing, were completely loose.
After fixing, the distances were gone and the print looks very good again.
The attached GIF shows the situation before fixing the bolts.