This error is seen after installing the Offline Client on a machine with Outlook 2007 installed and attempting to go into Cached Mode.
Outlook 2007 disabled the Add-In for the Offline Client
Perform the following:
1. Open Outlook 2007
2. Go to the Tools menu and choose Trust Center
3. Highlight Add-ins on the left
4. Choose Disabled Items from the drop down list at the bottom and click the Go... button
5. Highlight the Offline Client Add in and click the Enable button
