Just an update
I warrantied the defective pair and got store credit for them here on Altitude Sports that I used to purchased another pair of these same boots after being reassured by a store specialising in ski mountaineering equipment that also sells these boots that they never had any warranty claims for this same reason. I should have known better but they fit me so well that I wanted to believe that with my first pair I simply got an odd lemon. I was just putting them away for summer now and decided to check the shells. The right one already started to crack in the same spot as in my previous pair. Still quite subtle, probably because I used them maybe 6 times during this bad winter. Nevertheless, once it starts there is obviously no way to stop the crack from spreading. Be forewarned!