One-Page Checkout [Support Thread]

[lat9]

There's the issue:

jQuery version: 1.4.2  index.php:259:13

One-Page Checkout requires a minimum of 1.8.0 (or was it 1.7.0?) ... anyway, 1.4.2 isn't going to cut it.

You'll need to review your store's files (do you use Zen Lightbox, by chance?) to see where that version of jQuery is being loaded, remove it and (in your template's html_header.php file) load a more recent version of jQuery (like 1.11.0) for the One-Page Checkout (or any other plugin that uses an AJAX interface) to operate properly. You can use the admin Tools->Developers Tool Kit and search for 1.4.2.