ok guys no need to get angry at each other... so i think you all agree on one thing: it seems to be a software issue and not hardware related. right?
i already tried some higher extrusion multiplier settings (5%), but the print gets worse.

the filament i use did not come with the printer. i bought it, because i heard it is supposed to be good. i will try to measure the filament diameter and adjust it. i use sailfish and i think i didn't mention that i'm on a flashforge creator pro, but since those machines are basically the same i think it should make no difference.

i will also try to print some reference prints to see if the stepper motor settings are correct, but some step by step tutorial would be great