To reproduce:

public Form1()
{
    InitializeComponent();

    this.radScheduler1.ActiveViewType = SchedulerViewType.Day;
    RulerPrimitive ruler = (this.radScheduler1.SchedulerElement.ViewElement as SchedulerDayViewElement).DataAreaElement.Ruler;
    ruler.RangeFactor= ScaleRange.TenMinutes;
}

Workaround:

this.radScheduler1.GetDayView().RangeFactor = ScaleRange.TenMinutes;