Hi Viktor,
Great, that's solved my issue. I also had the problem of setting the MaxLength. That's also solved with your solution.
<telerik:GridNumericColumnEditor runat="server" ID="NumericEditor1">
<NumericTextBox runat="server" EnabledStyle-HorizontalAlign="Right" FocusedStyle-HorizontalAlign="Right" MaxLength="10"></NumericTextBox>
</telerik:GridNumericColumnEditor>
Thank you very much.