I HATE THIS ISSUE AND IVE FINALLY FIGURED IT OUT
For some reason this is an issue with your PCs default microsoft account. Go to settings>Accounts>Email & Accounts and then REMOVE ALL THE EMAILS AND ACCOUNTS ON THE ENTIRE PAGE. Basically whipe everything you can off this page so it is completely blank and void of any accounts or emails and dont have any accounts signed in to your windows/microsoft settings. Then open game bar and sign in.