Cannot close a modal jQuery-ui dialog

Question

I created a jQuery-ui based dialog, which contains an Obout combobox

<div id="dialogManufacturer" style="display: none">
    <table >
        <tr><td>Your choice: </td>
            <td>
                <obout:ComboBox runat="server" ID="cmbManufacturer" Width="100" Height="120" EmptyText="Please select a manufacturer ..."
                   Mode="ComboBox" FilterType="StartsWith"  >
                </obout:ComboBox>
        </td></tr>
    </table>
 </div>

Everything works fine as long as my dialog is not modal

 $("[id*=btnManufacturer]").live("click", function () {
    $("#dialogManufacturer").dialog({
        title: "Please select a manufacturer:",

        draggable: true,
        resizable: false,
        show: 'Transfer',
        hide: 'Transfer',
        minHeight: 10,
        minwidth: 10,
        width: 360,
        buttons: {
            "Ok": function () {
                var parameters = new Object();
                parameters.manufacturerId = getManufacturerId();
                parameters.FunctionName = "LoadManufacturer";
                ob_post.AddParam('parameters', parameters);
                ob_post.post(null, "DefaultCallback", JS_Callback);
                $(this).dialog('close');
            },
            "Close": function () {
                $(this).dialog('close');
            }
        }

    });
    return false;
});

but as soon as I add one property to the dialog

modal: true,

the dialog will be shown but I cannot close it. Neither "Ok" nor "Close" button close the dialog. The close cross-button in the right-upper corner of the dialog does not work too.

What could be a reason of such behaviour? Thank you.