Error message opening appointments

We just installed a new server running SBS2003. All client
workstations run Windows XP pro and use Outlook 2003. We
set up a shared company calendar. When users create
appointments they fill in the subject, location, start and
end time fields, and type a note in the text box. When
they save it and then try to go back in to view or edit
the appointment they get an error 'Can't open this item.
The operation failed.' This error only occurs when they
type a note in the text box. I've checked permissions and
created a new calendar to no avail. Any ideas?