[CalendarServer-users] Cannot delete an event?

The boss is running macOS 10.12.6 against Server.app 5.2 on macOS 10.11.6 The boss tries to remove a repeating event, but gets this error "The server responded: "403" to operation CalDAVWriteEntityQueueableOperation The event had two participants, the boss and another, however the event no longer appears on the other user's calendar. I don't know who was the originator of the event. Is there any way to force the removal of this event from the boss's calendar? Thanks Ian