The customer has a large infrastructure (thousands of mailboxes) running on a clustered Exchange 2010 Windows Server 2008 platform. The migration project completed and now all the users sit on these servers, and the legacy Exchange 2003 environment is empty and unused now except for some external routing. Many users report that they can no longer (since being moved from Exchange 2003) edit items in Public Folder calendars or mailbox calendars. They get the error ‘could not save item’ and ‘unknown error’ message. They also report slowness and freezing when try and edit other calendars. It was suggested to me that I should check the Event Viewer for events relating to the availability service, but there are none, and setting up perfmon with the 'MsExchange Availabilty Service Average Time to Process a Free Busy Request" does not show a problem i.e. readings should not be more than 5, but most of the time they are zero and do not move much from there. I really have no idea why this is happening. The Exchange 2010 servers were deliberately over-specced and are coping easily with the load. Can anyone suggest where I start in terms of troubleshooting? Thanks.