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Consider the following scenario:
You set up a Windows server cluster that has two nodes.
You install Microsoft Exchange 2003 on each node.
You use Microsoft Internet Security and Acceleration (ISA) Server 2004 to publish the Exchange Virtual IP by using the Exchange RPC Server protocol.
You move all resource groups from one cluster node to the other cluster node.
In this scenario, Microsoft Outlook clients cannot access the Exchange Server.
In the scenario listed in this article, the RPC response packets from Exchange contain a NULL context ID. ISA Server RPC filter rejects such packets because it expects the response context ID to match the request context ID. In some scenarios, it is legitimate for an RPC server to reset the context ID.
To resolve this problem, install the hotfix rollup package that is described in the following Microsoft Knowledge Base article:
970454 Description of the ISA Server 2004 hotfix package: June 2, 2009
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
For more information about software update terminology, click the following article number to view the article in the Microsoft Knowledge Base:
824684 Description of the standard terminology that is used to describe Microsoft software updates