Bert, can you set the ActiveSync logs to Detailed and when the next occurrence of this happens look in the logs for any Exceptions? You could also alternatively send the logs to email@example.com
attention Robert Emmett, and I would review them for any abnormalities. Additionally, is the appointment made on a personal calendar, shared calendar, or domain calendar? When sending the logs, could you also attach the organizer's calendarData.xml file? Thanks.