You can't clear the contents of a MaskEdBox control by setting the Text property to a null string if the MaskEdBox's Mask property contains delimiter. In fact, you must include those separators in the value you assign to the Text property, otherwise you get Error 380 - Invalid property value.
A simpler way to clear the contents of this control is to temporarily clear the Mask property, then assign a null string to the Text property, and finally restore the original Mask property, as in:
Private Sub cmdClear_Click()
Dim tmp As String
With MaskEdBox1
tmp = .Mask
.Mask = ""
.Text = ""
.Mask = tmp
End With
End Sub