I have read the earlier posts and was not able to find a definite answer, so
here goes again.
I have a user who is suddenly unable to create new appointments on his
default Calendar (the one in his Exchange mailbox).  Nothing I try brings up
the new appointment window - double-clicking, right-clicking, using the new
button, or using the file menu.
I can create a new calendar in his mailbox and add appointments to it just
fine.
We also use WebOutlook and I am able to create a new appointment on his
default calendar from WebOutlook.
We have a corporate version of Outlook and it does not have to be activated.
The problem persists even if the machine is rebooted.
I have been unable to find a solution and would appreciate any advice.