In eCart 6, the checkout page will already have the progress bar added to it, so adding it again will cause a duplicate and cause it to break.
The webflow/sparta/ express checkout URL Was a older beta URL for in context checkout, if you are using the most recent version of eCart to create the checkout pages, it will use the newer URL:
https://www.paypal.com/webscr?cmd=_express-checkout&token=EC-8H077254F16817303