I have heard about this happening very rarely. The most common cause I know of is if you try to save when the game's folder is located in a restricted folder on the PC (like "windows" or "program files"), and another one can be if the folder got set to "read only" somehow. More rarely, if a script got added/changed and you load an old save, sometimes things can go wrong too (but I don't think I changed any scripts in recent months).
Basically, it's a freak accident and more often than not it's not an issue with the game itself, so sadly I can't really "fix" it.