Calendar free/busy no info only for one employee

We have Exchange 2000 in the background and Outlook 2000 as email client.
Only for one user the free/busy scheduling does not show when she tries to
use the Attendee availability tab. I log in with another mailbox on her
machine and it is OK and I log in with her mailbox on another machine and it
is OK too. How can I reset this information for her or what is the solution?
Thank you.