The Exchange Team did a change in CAS to CAS proxy behavior between servers running Exchange 2010 SP2 RU1 and servers running older versions of Exchange.
The SP2 RU1 package introduced a change to the user context cookie which is used in CAS-to-CAS proxying. An unfortunate side-effect is a temporary incompatibility between SP2 RU1 servers and servers running earlier versions of Exchange. The change is such that earlier versions of Exchange do not understand the newer cookie used by the SP2 RU1 server. As a result, proxying from SP2 RU1 to an earlier version of Exchange will fail with the following error:
Invalid user context cookie found in proxy response
The server might show exceptions in the event log, such as the following:
Event ID: 4999
Log Name: Application
Source: MSExchange Common
Task Category: General
Level: Error
Description: Watson report about to be sent for process id: 744, with parameters: E12, c-RTL-AMD64, 14.02.0283.003, OWA, M.E.Clients.Owa, M.E.C.O.C.ProxyUtilities.UpdateProxyUserContextIdFromResponse, M.E.C.O.Core.OwaAsyncOperationException, 413, 14.02.0283.003.
Not all customers are affected by this. But since we received a few questions about this, we wanted to let you know about the change. Many Exchange customers do not use proxying between Exchange 2010 and Exchange 2007 but rather use redirection, which is not affected by the change. However, if you are using CAS-to-CAS proxying, where an Exchange 2010 SP2 RU1 Client Access server is proxying to an earlier version of Exchange 2010 or Exchange 2007 Client Access server, then you are affected by the change.
If you are affected, it is important to note that this issue is temporary and will exist only until all of the CAS involved in the CAS-to-CAS proxy process are updated to Exchange 2010 SP2 RU1. Thus, if you are affected by this problem, simply deploy SP2 RU1 on the relevant Exchange 2010 servers and the issue no longer exists.
If you use CAS-to-CAS proxy between Exchange 2010 and Exchange 2007, we will have an interim update (IU) for Exchange 2007. Availability of the IU will be announced on this blog.
Server proxy version
Server being proxied to
Action to take
Exchange 2010 SP2 RU1 –> Any version of Exchange 2010 older than SP2 RU1
Apply Exchange 2010 SP2 RU1 to all servers involved in proxy process
Exchange 2010 SP2 –-> RU1 Exchange 2007
Hold off deployment of Exchange 2010 SP2 RU1 until you deploy the Exchange 2007 interim update (IU)
SOURCE: Exchange 2010 sp2 ru1 and cas to cas proxy incompatibility