Hi everyone,
so I checked the belt tension and tightened the long axis belts. That didn't close the gap so I checked the motor belts, tightened them and BOOM -> perfect results :-)
In case you are curious what I was printing, I needed a webcam mount for my new screen. The border of the screen is too thick for the Webcam to grab it. On the picture you can see the result.
Thank you very much for your fast answers!
Matthias