To reproduce:
public RadForm1()
{
    InitializeComponent();
    date = DateTime.Now;
}
int dayCounter;
Random rnd = new Random();
DateTime date;


Timer timer = new Timer();
protected override void OnLoad(EventArgs e)
{
    base.OnLoad(e);
    LineSeries lineSeria = new LineSeries();
    DateTimeContinuousAxis continuousAxis = new DateTimeContinuousAxis();          
    continuousAxis.LabelFormat = "{0:dd}";
    lineSeria.HorizontalAxis = continuousAxis;
    radChartView1.Series.Add(lineSeria);

    for (int i = 0; i < 500; i++)
    {
        radChartView1.Series[0].DataPoints.Add(new CategoricalDataPoint(rnd.Next(1000), date.AddDays(dayCounter++)));
    }

    timer.Tick += timer_Tick;
    timer.Interval = 200;
    timer.Start();
}

void timer_Tick(object sender, EventArgs e)
{
    foreach (CategoricalDataPoint point in radChartView1.Series[0].DataPoints)
    {
       // point.Value = rnd.Next(1000);
        point.Category = date.AddDays(dayCounter++);
    }
}

Workaround:
void timer_Tick(object sender, EventArgs e)
{
    DateTimeContinuousAxis continuousAxis = ((LineSeries)radChartView1.Series[0]).HorizontalAxis as DateTimeContinuousAxis;

    HybridDictionary hashSet = typeof(Axis).GetField("hashSet", BindingFlags.NonPublic | BindingFlags.Instance).GetValue(continuousAxis) as HybridDictionary;
    hashSet.Clear();

}