When you proxy Exchange Control Panel (ECP) from Microsoft Exchange Server 2013 to Exchange Server 2010 Service Pack 3 (SP3) or an earlier service pack, the MSExchangeECPAppPool application pool crashes and generates a System.NullReferenceException exception.
Note This issue does not occur in Update Rollup 1 for Exchange Server 2010 SP3 or a later update.
Additionally, the following events are logged in the Application log in Event Viewer:
This issue occurs because proxying from Exchange Server 2013 does not authenticate to the target Exchange 2010 server for the resources in event ID 1309. Exchange Server 2010 SP3 and earlier service packs require authentication for every proxied request.
To resolve this issue, install Cumulative Update 7 for Exchange Server 2013.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.