The following procedure may work around this authentication issue:
1.) Go to the CMN Free Busy Connector server.
2.) Open "services.msc"
3.) Locate the "Quest Coexistence Manager for Notes Exchange Free/Busy Connector" service.
4.) Right-click on the service and select Properties.
5.) Click the Log On tab.
6.) Enter the credentials of an Exchange service account.
7.) Click OK.
8.) Restart the "Quest Coexistence Manager for Notes Exchange Free/Busy Connector" service.
9.) Create a new meeting in Notes and try looking up free busy data for an Exchange user.