Sorry for the delay. Unfortunately, the calendar is currently not fully supported. We did some tests and these fields - All day event and Recurrence, are indeed blank.
But we couldn’t get the error with URL, New Items are saved normally, as well as edited items. The only difference I see is the Name field, is that a Person field? Are there any other modifications to the default Calendar?
Since Calendar list currently only supports classic UI, you can check out our Forms Designer app, it has full calendar support.