Thank you for your answer! The original model was created in TinkerCad, meaning the resolution is really low, so this shouldn't be a problem.
Setting color scheme to "line type" is really helpfull, thanks for the tip! It seems the problem is with the travelling:
It is vissible, that the printer shortens the way where it travels on few spots - and those are actually the spots where we have problem afterwards:
Any idea how to make the printer to take the same path every time?
Thank you!