Assistant cannot delete appointments from permissioned calendar

I have an Outlook 2003 SP2 user who has Owner permissions to another user's
calendar that she manages. She can do everything in this calendar except
delete items - when she attempts to delete an item, she receives the error:
You do not have permission to create a message in this folder. She created a
test appointment and went back to delete it and received the same error.

We have removed her perms and readded them, tried all the other permissions,
to no avail.

I have other users with the same permissions or even Editor permissions (not
to the same calendar), and they have no issues managing other user's
calendars.

Thanks in advance!!!