Thanks for your idea. This greatly improves the z-Axis of the prints, as you can see in the following photo. On the left a print before replacing the motor holder, on the right side the results with the new holder. No replacement of the axis/motor shaft coupling. In the middle a M5 normal screw.
I redesigned your holder in openSCAD because i want to use M3 nuts and bolts to assemble the two parts. The openSCAD files can be found on thingiverse.
Again, thank you edirol for this big improvement of the printer.