The issue is when you drag up or drag down the top or bottom of the appointment i.e extend or decrease the slot.
The error I get with that same codebase is
Object reference not set to an instance of an object.
inner exception null

Source: Telerik.WinControls.Scheduler

StackTrace:
at Telerik.WinControls.UI.DayViewAppointmentsTable.appointment_MouseUp(Object sender, MouseEventArgs e)
   at Telerik.WinControls.UI.DayViewAppointmentsTable.AppointmentMouseUp(Object sender, MouseEventArgs e)
   at Telerik.WinControls.UI.RadSchedulerElement.scheduler_AppointmentElementMouseUp(Object sender, MouseEventArgs e)
   at Telerik.WinControls.UI.RadScheduler.OnAppointmentElementMouseUp(Object sender, MouseEventArgs args)...