"The HTTP request is unauthorized" error in a SharePoint Online calendar after you configure a calendar overlay to Exchange Online

PROBLEM

Consider the following scenario:
  • You have a calendar overlay in Microsoft SharePoint Online.
  • You configure the calendar overlay to do the following:
    • Retrieve a calendar from Microsoft Exchange Online
    • Display the calendar within the SharePoint Online calendar
In this scenario, information from the Exchange Online calendar isn't displayed in SharePoint Online. Additionally, you receive the following error message, which is displayed above the calendar: 
The HTTP request is unauthorized with client authentication scheme 'Ntlm'. The authentication header received from the server was 'Basic Realm=""'.

MORE INFORMATION

This issue occurs because the ability to use the calendar overlay in a SharePoint Online calendar that is retrieved from Exchange Online isn't supported.

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Article ID: 2997170 - Last Review: Dec 20, 2016 - Revision: 1

Microsoft SharePoint Online, Microsoft Exchange Online

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