outlook 2003 calender permissions downgraded but creates problems

User was permitted author permission to another user's calender and then down
graded to reviewer. Now the user is receiving warnings they can not create
item in the other calender or receives email meeting is not in other user's
calender. The schedule meeting is in the other user's calender. Summation:
User is able to schedule meetings but receives pop ups or email warnings.
User does not have permission to the mailbox but the user can view the other
user calender with no problems.