Can't access Outlook 2007 Calendar

We use Oulook 2007 with Exchange Server 2003.  I have a user who all of a sudden cannot open his calendar.  Both his administrative assistant and myself can open on our computers.  When he clicks the calendar icon, he gets the following message:

The connection to Microsoft Exchange is unavailable.  Outlook must be online or connected to complete this action.

This is happening inside the office from a desktop, not a remote connection, so Outlook is always connected when the server is running.  We run Outlook in cached mode, and his is connected.  He is getting new e-mail (without hitting send/receive), and can access all other aspects of his Exchange account.

I tried deleting the .ost file to see if that would help, but I am still getting the error message.  I have not tried repairing Outlook (this has never worked for any problem that I have had in the past, so I rarely give it a try).  I also have not tried reinstalling Outlook.

Has anyone run across this, and if so, what did you do to fix it?

Thanks in advance,

Craig