Consider the following scenario:
You have a calendar overlay in Microsoft SharePoint Online or SharePoint Server.
You configure the calendar overlay to do the following:
Retrieve a calendar from Microsoft Exchange Online
Display the calendar within the SharePoint Online or SharePoint Server calendar
In this scenario, information from the Exchange Online calendar isn't displayed in SharePoint Online or SharePoint Server. 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=""'.
This issue occurs because the ability to use the calendar overlay in a SharePoint Online or SharePoint Server calendar that is retrieved from Exchange Online isn't supported.
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