changing the section using XMLHTTP to using MSXML2 gets me a little further to fail at "unknown error! thumb.aspx does not work correctly":
//var xh=new ActiveXObject("Microsoft.XMLHTTP");
var xh=new ActiveXObject("MSXML2.ServerXMLHTTP");
xh.setOption(2, 13056);
xh.open("POST",url+"?job="+escape(jobfile),false);
xh.send("");
FAIL :-(