I figured it out. The slice is not aligned with the artboard (see screenshot1). Furthermore some slices are 1 pixel wider and taller than the artboard they belong to. But I never changed their size.
I cannot remember an editing step which would lead to this misalignment. After some tries and preparing the file for the upload to this page, I figured out that the slice-area is not linked to the artboard. Meaning, if I create an artboard and then move it, it's slice is somewhere else:
Am I handling the slices wrong? If nobody else is suffering this problem it must surely be me.
export_problem.afdesign