Hi Sander,
I really appreciate your help!
Alright, I cleaned the nozzle, the first run did remove some dirt apparently, the second was clean, no more residue on the filament piece. Then I printed one more calibration cube (guess what, I have about 20 of them sitting here on my desk) - no change, there's still a gap in the very same place.
I tried something: I ran Pronterface (the version included with Cura 15), moved the platform way up and then ran sequences of G01 Z30 and G01 Z-30 commands to slowly move it up and down, up and down. I've made a video where you can hear that there's something non-linear in the movement:
Maybe this helps find the reasons behind the problem?
Regards, Phil