If the type of the XAxis is category the field from the data source used for the XAxis must not be tried to be parsed to DateTime format.

For the time being the issue can be workaround as follows:

-Either set an appropriate DataFormatString for the XAxis labels. For example:

        <telerik:RadHtmlChart ID="ColumnChart" runat="server" Width="600" Height="400">
            <PlotArea>
                <Series>
                    <telerik:ColumnSeries DataFieldY="yValues">
                    </telerik:ColumnSeries>
                </Series>
                <XAxis DataLabelsField="xLabels" Type="category">
                    <LabelsAppearance DataFormatString="d - MM"></LabelsAppearance>
                </XAxis>
            </PlotArea>
        </telerik:RadHtmlChart>

- OR add the XAxis items manually from the data source:

ASPX:

        <telerik:RadHtmlChart ID="ColumnChart" runat="server" Width="600" Height="400">
            <PlotArea>
                <Series>
                    <telerik:ColumnSeries DataFieldY="yValues">
                    </telerik:ColumnSeries>
                </Series>
                <XAxis DataLabelsField="xLabels" Type="category">
                    <LabelsAppearance></LabelsAppearance>
                </XAxis>
            </PlotArea>
        </telerik:RadHtmlChart>
C#:

    protected void Page_Load(object sender, EventArgs e)
    {
        ColumnChart.DataSource = GetData();
        ColumnChart.DataBind();

        DataTable currDT = GetData();
        for (int i = 0; i < currDT.Rows.Count; i++)
        {
            AxisItem currItem = new AxisItem(currDT.Rows[i]["xLabels"].ToString());
            ColumnChart.PlotArea.XAxis.Items.Add(currItem);
        }
    }

    protected DataTable GetData()
    {
        DataTable dt = new DataTable();
        dt.Columns.Add("yValues");
        dt.Columns.Add("xLabels");

        dt.Rows.Add(10, "1 - 10");
        dt.Rows.Add(30, "2 - 11");
        dt.Rows.Add(20, "3 - 12");

        return dt;
    }