To reproduce:
  public Form1()
        {
            InitializeComponent();

            this.radMaskedEditBox1.Mask = "00,000-000";
            this.radMaskedEditBox1.MaskType = Telerik.WinControls.UI.MaskType.Standard;
        }

        private void radButton1_Click(object sender, EventArgs e)
        {
            radMaskedEditBox1.Value = null;
        }

Workaround:
        private void radButton1_Click(object sender, EventArgs e)
        {
            radMaskedEditBox1.Value = null;
            radMaskedEditBox1.Text = "";
        }