Calendar sharing keeps getting deleted - need help!

[SWO]

We use Outlook 2007 and share our calendars in our office. I had permission to view one of my co-worker's calendars for several months, but now when I try to log on on, a message pops up that says "The set of folders cannot be opened". I sent ther another permission request which she accepted again and allowed me to view her calendar, but the next day, I got the same "The set of folders cannot be opened" error message. Her name still shows up on under "Other People's Calendars" on the menu screen, but I get the above error message when I try to get into her calendar. Can anyone help?