There is difference between custom content and mods. Mods change some aspect of the game or add something like a gardener etc. Mods effect how the game works.
Custom content is are things created by talented Simmers who either clone something and recolor it or create a whole new item like cloths, hair, freckles, furniture and etc.
There is some bad CC floating around and that error you having is related to CC. (I had bad hair and freckles screw up my game.) I don't know how to make CC or mods. I just know some may work just fine for someone and not work in for others.
I do not know how it gets corrupt. I recommend that you stop using CC until you research all your CC and find out which CC is causing the error.
Take it all out and do a repair on your game. Google each piece of CC you have and see if others have reported problems or it has been updated. (they fixed some bugs on Nov. 25th I think and that alone could have corrupted your CC.)
Everytime I download something I put the URL into a folder in my favorites on Chrome so I can keep track of where I got it.
I also back up my game before I add anything. This way I can put my working files back if something goes bonkers.
