The problem is in the EventArgs of the AppointmentDelete handler:
sender, Telerik.Web.UI.SchedulerCancelEventArgs e)
The correct event args of the OnAppointmentDelete
event are AppointmentDeleteEventArgs.
I just took a look at our help article and I think that it misleads you. There's an error in our help article
and we'll do the needful to fix it.
the Telerik team