PROBLEM SOLVED!!!

I pried of the pinheader for D8 and D9 on the Arduino and discovered, that the metal-contact of D8 inside the pinheader had one half of the contact in D8 and one in D9, so it bridged the two pins.

A faulty pinheader caused all of this.
I desoldered the header and added a new one. Now my printer runs like a beast :-)

I have flashed the firmware joop posted. I seems to work better than the original one!

Thank you to everyone, who tried to help