The Object could not be found - New appointment error on shared calendar Outlook 2007

One user in our office cannot make new appointments on another users calendar. They have no problem adding a new contact, editing existing appointments, copying an appoinment from their calendar to the other users calendar, deleting appointments. They are using Vista Business and Outlook 2007. I had them login to an XP workstation with Outlook 2003 and they can add a new appointment without an error. The error says "The object could not be found."

I have tried deleting the frmscache.dat in the FORMS folder but it doesn't work. I have also tried a new user profile on the machine with no luck.

I ahve triple checked that the permissiions are correct.