Since Bug 24704 did get closed without addressing the problems (wtf?!)... Here it is:
it is not clear that there should only be entered one category in the
field. In fact the heading over it says "Kategorien" (categories) but only
one field is visible. I saw one entering a comma-separated list in there...
Put a bullet (<ul>) or something in front of the cat fields to show that this
is a vertical list of cats.