Microsoft Exchange Team released a note on their blog about an issue in CAS-to-CAS proxy scenario due a new cookie introduced on the SP2 RU1 release which is not compatible with old versions.
If you are using CAS Proxy a possible solution is to upgrade the entire server chain to SP2 RU1 however if you have proxy to a 2007 version then you need to wait for IU (Interim update) which will be released in a near future.
Ps.: If your organization using redirection instead of CAS Proxy then you are not affected by this issue.
Here is the original post:
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:
Event ID: 4999
Log Name: Application
Source: MSExchange Common
Task Category: General
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