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"Invalid user context cookie found in proxy response" error message and an event ID 4999 occur when you use an Exchange 2010 SP2 RU1 CAS server as a proxy to an Exchange 2007 CAS server
Article ID: 2696649 - View products that this article applies to.
Consider the following scenario:
Additionally, an event that resembles the following is logged in Event Viewer:
Invalid user context cookie found in proxy response
Event ID: 4999
This issue occurs because of change to the user context cookie in Exchange Server 2010 SP2 RU1.
To resolve this issue, download and install Update Rollup 7 for Exchange Server 2007 SP3. For information about how to download and install Update Rollup 7 for Exchange Server 2007 SP3, visit the following Microsoft website:
For more information about Update Rollup 7 for Exchange Server 2007 SP3, visit the following TechNet website: