To reproduce:
- Set the mask like this:
private void RadForm1_Load(object sender, EventArgs e)
{
    radMaskedEditBox1.MaskType = MaskType.Regex;
    radMaskedEditBox1.Mask = "^[0-9]{6} [A-Z]{4} [0-9]{7}$|^[0-9]{5}$";
}
- Start the application, enter five digits and pres Tab.
- The error icon apperas.

Workaround:
- Set the Mask before the MaskType:
radMaskedEditBox1.Mask = "^[0-9]{6} [A-Z]{4} [0-9]{7}$|^[0-9]{5}$";
radMaskedEditBox1.MaskType = MaskType.Regex;