@ElsaAzrael First of all, you shouldn't need to sign into the launcher in order to download content. When downloading from the store website, you're not supposed to need to be signed into the launcher at all.
For the issue itself, try changing your password to something that contains only letters and/or numbers, no special characters. Then clear the IE-mode browser data within Edge, as described at the bottom of this page:
https://support.microsoft.com/en-us/microsoft-edge/view-and-delete-browser-history-in-microsoft-edge-00cf7943-a9e1-975a-a33d-ac10ce454ca4
If you're still asked to sign in though, please try a new admin Windows account, just as an experiment. I don't know whether it would make a difference, but you're one of a few players who's reported this issue, so I'd like to find out. Make it a local account, as in, don't sign into Microsoft, and don't sync with any other services you normally use. The username should contain only letters and numbers, no special characters or accents.
If you can in fact download your store content, go ahead and download everything you want. Then go into Documents > Electronic Arts > The Sims 3 > Downloads and move all the .Sims3Pack files inside to This PC > Users > Shared. You'll be able to access this Shared folder from your original account, so you can grab the store stuff and transfer it to ...The Sims 3 > Downloads in that account, then install it through the launcher.