Can't log in to my account in launcher or game but can on the Sims 3 Store

abistasiasims  This isn't necessarily about duplicate files, but the problem here is that I don't have anything to test with at the moment.  (I just did a clean uninstall on one Windows setup and have a Steam install on the other.)  What you can do instead is try uninstalling the game with Revo Uninstaller, the free version, and reinstalling it.  Revo should find the same files.  Here's a guide in case it helps:

[Read First] Running Sims 3 in Windows | EA Forums - 8323446

You don't need to uninstall the EA App, and you can reinstall Sims 3 to the same folder you used before.

If you don't want to do that, I can put together a new install with some login and Store info and see what I find myself, then list the files I think you should try deleting.  But it's going to take a while—I have a lot on my plate right now.

Done! I relaunched the game and I'm still having the same problem.

abistasiasims  Just checking, but are there any special characters in the username of your main Windows account?  This isn't a common problem, as far as I've ever seen, but it's certainly possible that that could break things.  Even if there's a space, let me know.  You don't need to tell me your username if you don't want to; just say whether there are any characters aside from those in the Roman alphabet.

My username for Windows is my first and last name with a space in the middle. My email has a period in it though if that would affect it as well.

abistasiasims  The period in the email address is probably fine, or else we'd have heard about it before.  It's a common-enough move.  I do wonder about the space though.  You could test in another new admin Windows account (that you could delete when you were done), creating a new username similar to your main one, like lastname firstname with the space.

I realize this is a lot of testing, but at the very least, you can stop doing so whenever you want, for the moment or permanently, and play in the new account.

Just tried this and it was working there as well.

Update: I have no clue how this happened, but I can now log in on the launcher and in-game on my main Windows account. However, I get the "A serious error has occurred while loading [world name]. It is strongly recommended that you restart the application." when I try to load any world (including Sunset Valley).

abistasiasims  I guess that's a good thing overall, and perhaps the problem was something in temp files that Windows reset or replaced after a certain amount of time, like how browser cookies expire.

The "serious error" issue has its own master thread here:

[Current Issue] Sims 3 "serious error" when loading save | EA Forums - 8322938

The problem is usually OneDrive, and there are instructions on how to deal with it in the first post.