Forms inside tabs are null in IE 10+

[rfsoftware]

I have some multiple forms in a a easytab tab and in IE10 I can't access this forms, as they are returned as null. So this event code will not work as $(this.form) returns null. With easytabs disabled, all works normal and correct.

$(function() {
  $(".submit_order").click(function(){
    $.ajax({
      type     : "POST",
      cache    : false,
      url      : "AddItem",
      data     : "charset=iso-8859-1&doreload=0&" + $(this.form).serializeLatin(),
      success: ShowFancyBoxSuccess,
      error: ShowFancyBox
    });
    return false;
  });
});

With Firefox/IE9/Chrome everything is fine, but with IE10+ it fails as "this.form" is null. Any idea what is going on here?