Error Microsoft.Exchange.WebServices.Data.ServiceRequestException: The request failed. The underlying connection was closed: The connection was closed unexpectedly (4278242)

This error can occur if there is a Load Balancer in the environment. The load balancer may interfere with the connection staying alive when MAgE is trying to process large files or folders in a mailbox.. 

The timeout may be reproduced natively by launching EWSEditor on the MAgE agent host machine and trying to access the same folder/item that MAgE is failing to process. 

To prevent the load balancer from timing out any connections, bypass the load balancer and point the Virtual IP (VIP) for the load balancer directly to a CAS server in the hosts files on all MAgE agent host machines. This will ensure that all MAgE traffic from the agent host machine bypasses the load balancer and goes directly to a CAS Server. 

For these steps and for more information on how load balancers can affect MAgE traffic, please see the following KB article on MMEX agents and Load Balancers: https://support.quest.com/migration-manager-for-exchange/kb/101780