Error message when saving appointment if changing reminder
 

We have an exchange server and Outlook 2003 client. Something weird
that doesn't happen to anyone else.

Create an appointment by just double clicking the calendar, type topic
test and change the reminder to 5 days or anything else that is not in
the list.

When saving I get: "The item could not be saved because it has been
changed by another person. Do you want to save a copy in your default
Calendar folder?"

If I click yes, the appointment goes to my calendar just fine and the
reminder works. Why am I getting that message? Id I choose from the
reminders list, I don't get the message.