title: Outlook / Office Add-in fails with error: AADSTS900561: The endpoint only accepts POST requests. Received a GET == SOLVED
body: <hr>
  
  <p>We currently have an Office 365 Outlook add-in deployed in our environment that authenticates against Azure Active Directory.  In testing a Windows 10 (1909) upgrade with Office (1908) client installed, we encountered this error that rendered our add-in useless.</p>
  
  <p><strong><em>AADSTS900561: The endpoint only accepts POST requests. Received a GET request.</em></strong></p>
  
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  <h2>Issue:</h2>
  
  <p>Outlook add-in on (specifically) Windows 10 version 1909 with Outlook Office version 1908 cannot successfully load an add-in that uses SSO to authenticate against AAD.  Authentication fails.</p>
  
  <h2>Symptoms:</h2>
  
  <p>During the authentication process within the Outlook Add-in pane, Outlook pops out of the client to a new Edge browser window directed to autologon.microsoftazuread-sso.com and displays the following error: <em>AADSTS900561: The endpoint only accepts POST requests. Received a GET request</em>.</p>