In my case the X Beam was about 0.8mm to long, so that it was not possible to mount the joiner plates to the carriages. I modifed the xy spacers, so there is 0.5mm more spacer on every side and no more problems with the provided x Beam. 

I also modified the round corner, that made this part really hard to print out of abs/asa. Now there are no more problems while printing.