Magento AJAX problem in IE9 64bit - Using Simple Configurable Products Plugin

I have a Magento which uses the simple configurable products plugin. For those of you that don't know, once you've chosen your configurable product options, it then uses an AJAX query to get the custom options of the simple product which makes up your configurable product selection.

The problem exists on this page.

When someone choose the simple product they want based on the pack size, it is supposed to get the custom option when lets them decide on colour choice.

This works fine in every single browser except IE9 in 64bit mode (which happens to be the browser my client uses!!!!)

I have already tried to force the document type using:

<meta http-equiv="X-UA-Compatible" content="IE=8" />

but to no avail in this 64bit version...

The plugin developer have done their best to ignore me, so I'm kinda stuck. Here's the AJAX call that the script makes - I wondered if it's perhaps something to do with the ActiveX Object not being loaded?

Product.Config.prototype.showCustomOptionsBlock = function(productId, parentId) {
    var coUrl = this.config.ajaxBaseUrl + "co/?id=" + productId + '&pid=' + parentId;
    var prodForm = $('product_addtocart_form');

   if ($('SCPcustomOptionsDiv')==null) {
      return;
   }

    Effect.Fade('SCPcustomOptionsDiv', { duration: 0.5, from: 1, to: 0.5 });
    if(productId) {
        //Uncomme开发者_运维百科nt the line below if you want an ajax loader to appear while any custom
        //options are being loaded.
        $$('span.scp-please-wait').each(function(el) {el.show()});

        //prodForm.getElements().each(function(el) {el.disable()});
        new Ajax.Updater('SCPcustomOptionsDiv', coUrl, {
          method: 'get',
          cache: false,
          evalScripts: true,
          onComplete: function() {
              $$('span.scp-please-wait').each(function(el) {el.hide()});
              Effect.Fade('SCPcustomOptionsDiv', { duration: 0.5, from: 0.5, to: 1 });
              //prodForm.getElements().each(function(el) {el.enable()});
          }
        });
    } else {
        $('SCPcustomOptionsDiv').innerHTML = '';
        window.opConfig = new Product.Options([]);
    }
};

Any thoughts/ideas would be much appreciated!

As Anton S suggested, upgrading Prototype actually worked! You can download the latest version of the framework from here: http://www.prototypejs.org/