@KyrCoe You shouldn't need to be signed into the launcher to download content. I never sign in unless I'm testing the function, and I can always download content just fine. If you're getting an error when downloading content, try deleting the file in Documents\Electronic Arts\The Sims 3\Downloads labeled [your email].bin.
For the launcher login issue itself, try clearing the cache for Internet Explorer. Even if you never use it, the launcher does for some purposes. Be sure to uncheck the box to preserve data for favorite sites.
https://kb.wisc.edu/page.php?id=15141
If that doesn't help, change your password to something with only letters and numbers, no special characters, accents, etc. Then clear IE's cache again.