Hi,
Not sure if you're still looking for an answer to this one, but it's something I came across this week.
The issue is that there is some Microsoft JavaScript on the Appointment entity that is referencing the "isalldayevent" event.
If this is not present on the form then you get a JavaScript error (and it also prevents the Appointment from being saved from my experience).
So, simply add "Is All Day Event" to your Form (make it invisible if required) and you should see that it solves the issue.
Best regards,
Andy