We've got this event migrating from Exchange 2003 to Exchange 2010 (on the Exchange 2010 server). Update Pack 4 for Exchange 2010 didn't fixed the problem. Users on 2010 couldn't see free & busy from users on 2003. I found the solution at EV100311
(Free/Busy Not Working Exchange 2007).
If you migrated users from an Exchange 2003 server, chances might be good that you did not migrate over the Free/Busy public folder information correctly. To check open the Exchange Management Shell (EMS) as administrator and run the following command:
Get-PublicFolder -server CASSERVER "\non_ipm_subtree\SCHEDULE+ FREE BUSY" -recurse | Format-List
You will see a bunch of information, but what you are looking for is two Public folder groups:
\NON_IPM_SUBTREE\SCHEDULE+ FREE BUSY\EX:/o=Company/ou=First Administrative Group - AND
\NON_IPM_SUBTREE\SCHEDULE+ FREE BUSY\EX:/o=Company/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)
In my case, we were missing the First Administrative Group. To add it I ran the following from EMS:
Set-PublicFolder –Identity "\NON_IPM_SUBTREE\SCHEDULE+ FREE BUSY\EX:/o=Company/ou=First Administrative Group" –Replicas "CASSERVER\Public Folder Database"