The response received from the service didn't contain valid XML: This error is generic in nature and indicates that, instead of the expected XML response (for example: SOAP web service response), we received either a) an HTML error page, or b) a truncated XML response.
- Make sure the specified end user email address is correct.
- Check to see if there is any restriction (i.e., a firewall) which could truncate XML responses larger than a certain size, or a load balancer/packet accelerator that is interacting with the network traffic.
- Check that EWS access works:
- Make sure that the EWS endpoint for the environment is accessible via the internet.
- Test EWS access on the Exchange server using PowerShell (i.e., Test-WebServicesConnectivity). More information on the PowerShell tests can be found in the following TechNet article: https://technet.microsoft.com/en-us/library/aa998328(v=exchg.160).aspx
- Test EWS access for an external connection.
Make sure that the Exchange servers within the environment are fully up to date with all required Service Packs and patches.
- Restart all Client Access Servers.
Review IIS logs and system Event logs.
- Check the .config file under the /EWS IIS directory. You can use a free XML validation tool, such as http://www.xmlvalidation.com.
- Check for duplicate XML nodes.
- Check for unclosed XML nodes.
- Check for references to local host names not reachable from the Internet.
- Check for httpRedirect XML nodes.
- Recreate the EWS Virtual Directory within IIS