New Billing Address Goes to New Page

When clicking on the "New" for billing address, the user gets redirected to www.ilovespeedskating.com/component/hika...lling/tmpl-component

And a Javascript error is thrown:
Error: this.element.getProperty is not a function
Source File: www.ilovespeedskating.com/media/system/js/modal.js
Line: 1

You should look at the "Problems with modal popups" documentation on that page:
www.hikashop.com/en/support/documentatio...tibility-issues.html

That sounds like it's your problem...