Blank screen with ‘Grant Office 365 permissions’

I have configured the client ID and secret for Office 365 OAuth option, (with the office 365 extension installed.) When clicking ‘Grant Office 365 permissions’ it just returns a blank page. If I try and send a test email I get ‘No access token has been provided.’. I have the latest versions of the Post SMTP plugin and Office 365 extension installed. What could be the issue? thanks