Bloomthewolf Are you getting the "serious error" message? When you turned off OneDrive, did you confirm that your Documents directory is no longer in the OneDrive folder, or that Sims 3 is no longer trying to write to that folder? This is the location I mean:
C:\Users\your username\OneDrive\Documents
The game should write to ... your username > Documents instead, but if it's still trying to use the OneDrive folder, that would explain the problem.
Another way to check is to ask Command Prompt for the location of the Documents directory:
reg query "HKCU\Software\Microsoft\Windows\CurrentVersion\Explorer\Shell Folders" /v "Personal"
The game will always try to use this directory if it can.