Can't save changes to existing calendar appointments

[Q90Partner]

I have a hosted domain where the COS has SHARING and GROUP CALENDAR turned off.

Through the WebClient, I can create and delete appointments, but when I edit any field of an existing appointment, I hit the SAVE button and nothing happens. I hit CLOSE and the dialog "Do you want to save changes" pops up. I say YES and the dialog goes away, but not the appointment page. I hit SAVE and nothing. I hit CLOSE again and the same dialog pops up. If I hit NO then the appointment page closes.

This problem goes away when I turn on GROUP CALENDAR, but these users aren't paying for group calendars. Maybe I'm misunderstanding what GROUP CALENDAR really is, but only when that's turned on can I edit existing events.

Something is preventing me from modifying appointments when GC is turned off.