If you have raised the diagnostics logging for Message access you may have the issue described in the new post of MSExchangeTeam blog.
There is potential for calendar related problems when new diagnostic logging for Message Access, which is available in Exchange Server 2007 Service Pack 2 (SP2) is raised from its default setting of Lowest.
A Knowledge Base article and a fix are in the works at this time and will be available soon.
Important - some calendar items may remain broken even after applying the update. This post addresses scenarios where those lingering symptoms remain.
Read more at source: http://msexchangeteam.com/archive/2010/01/11/453755.aspx