This is usually related to a server cache issue on our end. It will also appear if you recently deleted an Age of Empires forums account that you were previously signed in to. 

🛠 This issue will require reaching out to AOEInsiderSupport@microsoft.com for assistance. We will need your gamertag, the email address of the Microsoft account you are wanting to sign-in with, and a screenshot of the error (including the URL the error is appearing on).