During a migration from Exchange Server 2003 to Exchange Server 2007 you need to add the Exchange 2007 server to replicas for each of the Public Folders (as you would need with any Exchange server migration) and this includes the System folders as well.
In our case I missed the SCHEDULE FREE BUSY folder.
This event may occur if the public folder store, where Free Busy information is published, is in the Dismounted state.
ADSI Edit can be run from a client computer or server.
These problems may require you to reinstall Windows Server 2003, Exchange 2007, or both Windows Server 2003 and Exchange 2007.
The computer does not have to be a member of a domain, but the user must have the rights to view and edit the Active Directory directory service domain to which the user is connecting.