There is an issue during an EX2003 to 2010 migration. When an Exchange 2003 resource mailbox is migrated and then converted to an Exchange 2010 resource, clients get this error.
Error Message: "Resource Has Declined Your Meeting Because it Is Recurring"
This error occurs for Outlook 2003, 2007 and 2010.
When creating a new resource mailbox on 2010 servers there are no issues on Outlook 2003 / 2007 or 2010 clients - only migrated mailboxes have the problem.
This issue occurs if the Auto Accept agent is enabled on the Exchange 2003 resource mailbox. This needs to be unregistered before you move the Exchange 2003 resource mailbox to Exchange 2010.