I just encountered this problem 2 seconds ago.
What happened is I had tried to use ArchiveInvalidation invalidated to enable some mods, which subsequently broke my game, because it has a documented propensity to screw up the game on older computers (like mine). So i had to system restore and reinstall the game. Now, I reinstalled the exact same mods i had in this save file and activated them in the data files. I am using the save file from before i reinstalled. Nothing has changed, the state of the OS and game are exactly the same.
I have the exact same mods activated now and whenever the game saves, it immediately crashes. I have no idea why.
Also I'd like to congratulate a few people this post on their incredibly terrible input. He gave you the most specific information possible on the error in quite common English. It crashes when it saves. That means that whenever the game makes calls the save() method, the game crashes. Depending on your save settings that can be when you auto-travel, when you go through a loading screen, when you turn in a quest, or all of them. It has nothing to do with external influences such as background services, memory, etc. The issue revolves entirely around a runtime exception somewhere, and the hell if I know where because of course it won't tell you anything specific, but I'm about to get pissed. I've accomplished more in 15 hours playing this game on the PC than I did in a month of playing it on Xbox and actually beating the game, and I'm not going back and doing it again, whether or not I'm on break.