During an upgrade to service pack 2 of an Exchange 2003 active/passive cluster, we had the following error:

The solution was simple but difficult to find. You need to make sure that the cluster's public network interface is listed first in the network binding order. This is done from Control Panel / Network Connections / Advanced menu / Advanced Settings.

So before you start such an upgrade, check those settings and correct them. It is actually a best practice when a cluster is installed.