Hello and thanks for your replies!
You are right about the planes. The way I used them in my little sketch doesn't make much sense. I trained myself to use planes whenever possible. Is this a bad habbit? What do you think?
However, back to topic:
Thanks to @chrisb's example I think I was able to narrow down the problem and make my example even more "minimal" (please find it attached).
I added an octagon to Sketch001. Interestingly, the PolarPattern works when I drag the vertical line from Sketch001 (the only left degree of freedom) inside the octagon and it fails when I leave it outside (please try it out yourself).
In other words: the polar pattern only works when the "clones" of the Additive Loft overlap.
So I would like to ask again: is this a bug or am I doing something wrong?
By the way: With your help I was already able to solve my initial problem, so I only ask out of "academic" interest and because I want FreeCAD to get better and better