Quote Originally Posted by Nick1973 View Post
yes it is there Cindy

On this path /includes/classes/ajax/zcAjaxOnePageCheckout.php

On the two sites that are affected
Well, there's something else going on in the site. Perhaps check the .htaccess files. If that class-file is present, there's something specific to those sites that is resulting in that 404 ... which is ultimately preventing OPC's AJAX calls from working.