Unable to open calendar items in Outlook 2003.

[Kate]

I suddenly cannot open any items in my Calendar folder.
When I try to view items in the reading pane, I get the message
"This message with restricted permissions cannot be viewed in the reading
pane until you verify your credentials. Open the item to read the item and
verify your credentials."

No special permissions have been set, and I can open the items fine if I
move them to another folder, but not within the Calendar folder.

Also, if I try to copy multiple items to the calendar, only the first item
is moved.

Kate