"Create your account" button doesn't work in IE 8.0

[ebobnar]

The “Create your account” button on our login/registration page does nothing when clicked in IE 8.0.

We were able to fix it so the form submitted using the solution provided here:

http://www.prestashop.com/forums/topic/193839-create-account-problem-prestashop-15/?do=findComment&comment=1061112

However, when we made that change, we noticed that clicking the button now gives a “Technical error: unable to load form” warning in IE 11.0.

So we rolled back the earlier change and are back to that button not working at all in IE 8.0.

Is there any solution that gets that button working in both IE 8.0 and IE 11.0 (and preferably most other browsers too)?

We're running PrestaShop 1.5.3.1, and our store is at: http://www.perkinsproducts.org/store/en/