I also think the plastic diff cup is the culprit. I measured the ring gear today, and it seems to be completely flat and true.

The cup, on the other hand, seems to be a little warped. If I measure the outside diameter of the cup, it measures between 29.6mm and 30mm depending on where you measure it..
The inside diameter of the ring gear is 30mm and has a little corner radius, so when you screw the pieces together the sharp corner of the plastic cup actually rests agains the corner radius in the ring gear in some places.
Maybe putting a little chamfer on the cup will help solve the problem?
I'll give it a try tomorrow.