This happens because the events are no longer stored in the ViewState by default (since 6.2). I will fix the exception but if you leave your code as it is you will get an empty calendar.
You need to do one of the following:
- Call DayPilotCalendar1.DataBind() before calling DayPilotCalendar1.DataBind() [recommended]
- Set StoreEventsInViewState="true"