A few users had reported Outlook 2003 Shared Calendar issues when migrating to Exchange 2010. Although Outlook 2003 is “Supported” on Exchange 2010 there are a few issues. The most annoying one I came across was the inability to open more than about 5 shared calendars or mailboxes. Users get a misleading error “The connection to the Microsoft Exchange Server is unavailable. Outlook must be online or connected to complete this action.”

This is due to how Outlook 2003 connects to the exchange server. Each calendar opens a new RPC connection. Once the number of connections hits 20, the exchange server will block new connections, which means you need to close Outlook and then open it back up again to get into any other calendars.

There are 2 sides to fixing this problem

1. The server side. Create a new Throttling Policy and assign it to the problem users so they can make additional connections http://support.microsoft.com/kb/2299468
2. The Client side. Sometimes you will need to remove all the calendars from the profile and then re-add them to get rid of stale information. If there are still issues after this, the most likely cause is an AV plugin in outlook. These need to be disabled in the outlook profile.  ESET, Kaspersky and Trend all seem to cause this issue.