neoromanempire The "serious error" is typically permissions- or access-related, at least these days, but borked permissions or denied access can happen for a variety of reasons. OneDrive is simply the most common offender. The concept is reinforced in your case by the fact that the local account worked even if OneDrive isn't your specific culprit.
As for why this can happen when all other games that access Documents are fine, I can't be sure. But I do know that Sims 3 generates a LOT more temp data than the other Sims games, and much of that is generated as the game starts to load a save.
For the issue at hand, do you have anything else backing up Documents (iCloud or whatever), and does pausing that syncing help? I realize you might not want to do this long-term, but it would at least be helpful to know whether it made a difference.
Separately, I can suggest an experiment to you, if you're interested. Create a symbolic link for the Sims 3 user folder pointing it elsewhere on your drive, and specifically NOT in Documents or any folder you want syncing or that a syncing service might try to reach, and not in Program Files since it's somewhat protected by default. You can just create a new folder on C called whatever you want, and put the Sims 3 folder inside that. This guide was written for Sims 4 and an external drive, but it applies here too:
If you want to try this but need more specific help, please list the new folder, and let me know if your Documents directory is not in the default C:\Users\your username location, according to Command Prompt. I take it you found the relevant command already.
