Workaround: use your own icon: 

public Form1()
{
    InitializeComponent();
    this.radScheduler1.RecurrenceEditDialogShowing+=radScheduler1_RecurrenceEditDialogShowing;
}

private void radScheduler1_RecurrenceEditDialogShowing(object sender, Telerik.WinControls.UI.RecurrenceEditDialogShowingEventArgs e)
{
    ((RadForm)e.RecurrenceEditDialog).Icon = Properties.Resources.WinFormsIcon;
}