Hi Clemens
As others have said it is important to open and close the hood in the right order. OK it is in the manual but your dealer should have given you step by step instruction. I know I have occasionally tried to close the hood without releasing that particular fastening. Between my wife and myself we often talk the steps through as done in the right order it is an easy job - even in the rain!
One thing I have found out, having had three with the same hood, when putting the hood up doing the doing the second to last step of fastening down the rear spring catches is to stand directly behind the car in order to push with enough force and in the right direction.
Looking at my own car I guess that the fastening nipple mast be only 2.5mm diameter and with the thread depth would easily fracture off. My guess is it happened in the initial assembly and yes it will be a warranty issue. Don't try doing it yourself!
Kerry
