Backup, recovery, or other operations with Microsoft 365 items fail with the following error:
.NET exception 'System.Net.WebException' has occurred: The underlying connection was closed: An unexpected error occurred on a send
This applies both to C2C backup and to local Agent for Microsoft 365.
The error comes from Microsoft and indicates certain problems with the connection to the EWS service in the cloud. Similar errors have been reported in the Microsoft community: EWS request failed
The issue was fixed on Microsoft side and is not expected to reproduce after November 2021.
If you face such an issue, as a workaround, restart the backup task manually: several attempts may be needed. Splitting a large backup task into several tasks with a smaller number of items may also help.
For local Agent for Microsoft 365: change the backup schedule.