I was unable to access information in Tools | Options | Xamarin | Apple Accounts due to a login error. I duly went and accepted the agreement and continued to get the error (pictured below). I restarted Visual Studio (17.8.3) - no luck. I even removed my account - no luck: got the same error when adding my API key in again.
How to fix this?
It turns out the same error also appears when the App Store Connect site wants you to update other information. I hunted around and found I needed to enter in an address on the App Store Connect site under Agreements, Tax and Banking.
That fixed things right up.