Then it's probably something you downloaded and you need to find out what it is.
Use the 50/50 Method: http://simswiki.info/wiki.php?title=Game_Help:Finding_Problem_Custom_Content
OK, update: So when I rename my Sims 4 folder to backup and restart the game it works fine....and Origin throws up this "Downloads Complete" splash screen, although nothing downloaded. But, when I add my folders (mods, saves, etc) back to the new Sims 4, I get the same error. I'm going to start over and add each folder back one by one to see if I can isolate which folder it is that's causing the issue. I know it isn't mods because previously I tried starting the game without anything in the mods folder and got the same result. I'm just hoping it isn't the 'saves' folder. I'm going to be super angry if I've lost everything!!! I'll keep you posted with what I find!
Maybe it's just one file in the saves folder and the backups still work. Makes sure to try the saves one by one. To make a backup active, just remove the verX (x stands for a number between 0-4)
