S2 window keeps breaking while down in door

[wolfchen]

I have had my doors apart multiple times and keep having the glass crack while down, during drives in the Elan.
I don't see anything loose or protruding in the fittings and as I have gone through 4 sets of windows , I am at dead end. It seems that cracks radiate from the from the front lower edge . Any thoughts as to what I am missing ? perhaps the scissor mechanism is to tight ?