Sanquelle Try entering your username and password one time. Whether the prompt accepts your login data or not, this may get you past the message the next time around.
If that doesn't help, try a different network, if possible. For example, if your computer can use wifi, set your phone as a mobile hotspot, using data rather than your home network, just long enough to try to launch Sims 3. That shouldn't use much data, and after that, you can test your old network again, or put the EA App in offline mode before launching the game.
If that doesn't help, try playing in a new admin Windows account. Make it local, as in, don't link it to your Microsoft account or any email address. You'll be able to (try to) open Sims 3 without reinstalling it.