User is trying to view entries in the calendars of colleagues and gets the following error when sending a sharing request: "Error While Preparing to Send Sharing Message". 

I started Outlook in safe mode to check if add-ins were causing the issue but the problem persists in safe mode. Tried repair of Outlook with Microsoft Support and Recovery Assistant. Also reinstalled Office with the same result. 

This is a local installation of Outlook 16. Windows 10.

Any help would be appreciated