The screws for attaching the lower part are too small or simply bad, one broke off during assembly (of course pre-drilled) I replaced it with something stronger, so no problems. The price bears no relation to the 4 cm large plastic part.
After pre-drilling the holes, the head of one of the included screws simply snapped off while I was manually screwing it in. The plastic parts seem to be of high quality, but it looks like they skimped on the screws... In the end, though, everything is installed and serves its purpose – even with the broken screw.