Recurring Appointments

When modifying one occurrence of a recurring appointment
we are experiencing the following errors:

As soon as we select "Send Update", the following error
appears

The function cannot be performed because the message has
been changed.

However, the update is sent to the invitees but the
sender's calendar is not updated and the appointment
screen remains open. We next select "Save and Close" and
the following error appears

The item could not be saved because it has been modified
by another user or in another window.

So we next select the X to close the window and we get "Do
you want to save changes?" We say YES. And we get the
message "the item could not be saved..." again. We do
file/close and say NO to "Do you want to save changes?"
the window then closes. The calendar is not updated, but
the invitees get the update.

If we close Outlook and modify the appointment, it appears
to work fine. But it's not consistent.

Does anyone have a clue as to what is happening here?

Thanks,
Miriam