Be sure to call form's methods after window is opened. In your demo link
http://test.reems.develop01.goldenvalue.nl/realplan13.php, please use the code below to open your window.
function newrec()
{
$('#formtabs').tabs('select',0);
$('#w').window('open');
$("#rpf").form('reset'); // the reset method should be called after the window is opened
$("#rpf :input").attr("disabled", false);
$('#select1').hide();
$('#submit1').show();
}