I had a hard time figuring why my application was crashing so I share it here to avoid other people to make the same mistake.
To make it short, the fact is I was calling unintentionally some Multiple Touch Object functions while the object had been destroyed earlier in the code (Because it was intended for an android application the object was destroyed at start when testing it in windows runtime where it was useless - until I reactivated by mistake a group which contained calls to it and led to the crashes -).
I thought that destroying an objet by code would deactivate all the links to it like when you delete it straight from the frame. That's not the case, links are kept but pointing to a no more existing object.

Moral of the story : don't call functions on an object you destroyed earlier in the code -> crash
(and don't overdo optimization - though it was not very smart to destroy downright the object instead of just dealing with groups or conditions -)