I finally solved this.
Go into Settings->Accounts->Email & accounts and remove all Microsoft accounts. If there is not a Remove button, see https://answers.microsoft.com/en-us/windows/forum/all/no-remove-button-for-microsoft-account-in-windows/bda1598b-ee49-435e-877a-62aed90d149c
Now, go into your app (i.e. Microsoft Store, etc.), select Sign In->Use a different account. Enter your Microsoft account login and password.
I removed and re-added 3 Microsoft accounts this way. All now work - no more blank, white screen.