Hey guys, this is a long over due update with regards to http://blogs.msdn.com/dgoldman/archive/2008/02/02/looking-up-free-busy-information-in-a-mixed-org-e2k3-e2k7-will-return-a-4004-error.aspx.
For more information please see: KB Article 972076 (http://support.microsoft.com/kb/972076/ ) Description of Update Rollup 2 for Exchange Server Service Pack 2
To resolve this problem, follow these steps: Install Update Rollup 2 for Exchange Server 2007 Service Pack 2 on the Exchange server.
1. Locate the Web.config file in the following folder:
2. Open the Web.config file in Notepad, and then locate and add the following line between the <appsettings> element and the </appsettings> element: <add key="UseLocalReplicaForFreeBusy" value="true" />
NOTE: If the value of the key "UseLocalReplicaForFreeBusy" is true, Exchange Server 2007 CAS checks if there is a replica for the Exchange Server 2003 users on a local public folder server. If there is a replica available, Exchange Server 2007 CAS uses the replica instead of a low bandwidth WAN connection.
3. Save the Web.config file and then restart the Exchange Web Service on the Exchange Server 2007 server.
4. Do the same change in the Web.config file located in: %ProgramFiles%\Microsoft\Exchange Server\ClientAccess\OWA