Originally Posted by _N_
Yeah, the seat is twisting in the frame.
I'll try tightening it even further, it has the quick release system. If that doesn't work I'll have to take it back to the shop. I tought that there might be a trick to it to get it to stay in place.
If you just bought it, go back to the shop and let them take care of it. If you tighten the seatpost clamp to much (usually this is mainly a problem if you have a bolt on clamp and not a quick release clamp) you can deform the seat tube of the frame. As mentioned before, check (or have the store check) the seatpost diameter and the seat tube inner diameter.