I'm a fan of Thomann, but the design and construction of this piece is frankly ridiculous ! As others have pointed out, the 5/8 female threaded ring is just lightly pressed into the plastic part, so it pulls out easily - potentially causing your beloved personal monitor to crash to the floor. There's also nowhere near enough thread.
However, alternatives are hard to come by, so for a bit of effort it can be improved. I used a 5/8-F to 1/4-M adaptor and extended this with a piece of 6mm brass tubing, reamed out to 1/4-20 and screwed tightly to the adaptor (with thread-lock). I then pressed this piece into the plastic part, packing this first with epoxy so it won't pull out.
Ridiculous to have to do this, but I think it will hold !