Haven't been able to play The Sims 3 since i built my new pc in december

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:

https://crinrict.com/blog/2020/02/moving-windows-documents-folder-to-external-drive-via-symbolic-link.html

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.

I created the symbolic link and the link worked but it didn't change anything, I still get the error message trying to load a game.

neoromanempire​  Is your Documents directory in C:\Users\your username\Documents or somewhere else, and do you use another syncing service?

Yes it is in there and I have icloud but it doesn't sync anything on my pc except photos.

neoromanempire​  Are there any special characters in the name you use for your Windows account?  This isn't a typical problem with Sims 3, but it could in theory have some effect, and this would also explain why the new account worked.

If you're willing, you could also try moving the location of Documents elsewhere, to see whether the problem is that folder.  You could also reverse it after the fact if you didn't want to keep it where it was, whether moving it worked or not.  As in, this can be considered an experiment and possibly a workaround, or not, depending on your preference (and whether it helps of course).

The normal way to move Documents is this (option one):

https://www.elevenforum.com/t/move-or-restore-default-location-of-documents-folder-in-windows-11.8708/#One

You can also force a new location with a registry edit:

https://support.microsoft.com/en-us/topic/operation-to-change-a-personal-folder-location-fails-in-windows-ffb95139-6dbb-821d-27ec-62c9aaccd720

the new location just copied all the folders into a new folder and it kept the originals in my documents folder. I dont even think they're using the new ones.

neoromanempire​  What is the new location though, according to the registry; and does it get around the problem to have Documents in a different location?  It doesn't matter whether anything is copied around; the point is what Windows thinks it should use to store Documents-related data, and therefore what Sims 3 is supposed to use as well.

I'm not sure how to check that, what should I do? I made a new folder in :C drive and changed the documents location to that but it changed nothing except copying the folders to there and it seems to still be using the originals in the documents directory.

neoromanempire​  Two things:

You changed the value in the registry, so you can check the registry again to see whether that value stuck, and if so, let me know what it is.

You can rename any "The Sims 3" folder on your computer that has a DeviceConfig file inside, to anything you want, then open the Sims 3 launcher and see where the new Sims 3 folder gets created.

I already undid moving the documents. So I should do it again and then check the Registry? How do I check the registry exactly?