Wow , thats really detailed and helpful.
But you don't have to be sorry this is not really a run around compared to other bugs that appeared after Game Maker Updates and took weeks to solve because they had nothing to do with my code and even if it was a run around, it's my fault because this bug has most likely to do with the way I programmed the savesystem.
I can't tell if it is rare. I couldn't recreate it but that doesn't mean it won't happen to others. I really hope it is rare.