The response received from the service didn't contain valid XML. (4317287)

When using the MAgE (Migration Agent for Exchange) the following error is displayed for failed mailbox synchronizations.

M16    Info    Ews Connection. Url: 'https://server.domain.com/EWS' Username: 'srviceaccount' Domain: 'Windows_Domain' Mailbox: 'usermailbox@domain.com' PreferredCulture: 'en-US'
M16    Trace    EWS:  Connect end
M16    Trace    EWS:  StartSync end
M16    Error    Microsoft.Exchange.WebServices.Data.ServiceRequestException: The response received from the service didn't contain valid XML.

There are 2 causes for this currently.

1. The EWS URL is incorrectly configured.

2 There is a load balancer in between the Agent Host and the Exchange server.

To resolve the error message follow the below resolutions for the numbers in the "Cause" field.

1. Open up the Exchange Control Panel and add the "/Exchange.asmx" to the WebServices internal/external URL

2. Modify the host file of the Agent Host to point directly to the CAS Server.