We wanted to give you a heads up regarding 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:

--> Please see the rest on the blog : http://blogs.technet.com/b/exchange/archive/2012/02/17/exchange-2010-sp2-ru1-and-cas-to-cas-proxy-incompatibility.aspx