User 1 has permission to view User 2's calendar and can do so successfully.
However, after a little while it stops working. When User 1 tries to view
User 2's calendar, he gets a message saying "Unable to open folder". This
cannot be made to work again until the permission for User 1 on User 2's
computer is removed and re-added and the cycle begins again.

User 1 does not have this problem with anyone else's calendar, and no-one
else has a problem viewing User 2's calendar.

Any ideas?

Steve