Found the problem! RS72 put me on the right track with his picture of the indexing mechanism. (thanks Ramon!) The culprit was the black plastic part that is located inside the spring in the "R2D2" knob sticking out on top.
Mine was broken (!?!) and the broken off sliver was lodged in the housing probably blocking the whole mechanism from moving up and down as designed. Managed to glue the 2 pieces back together and made some reinforcements with CA glue in combination with baking soda trick we use in RC. So it holds up for now, but I'll contact the vendor to get a replacement.