The problem occurs when an Outlook 2010 user gives free/busy permissions to an Outlook 2007 user. The Outlook 2007 user is unable to open calendar and receives following error message: You don't have sufficient permissions to open the calendar folder.
This is a known issue. The behavior is different for Outlook 2010 and 2007. Outlook 2010 will allow you to open a calendar to view the freebusy but Outlook 2007 will not unless you explicity have at least reviewer rights; so with Outlook 2007 you typically use the scheduling assistant to view free busy.
Workaround: Outlook 2010 user has to give at least reviewer rights to the Outlook 2007 user.