Test account credentials received! The cause of the message, I believe, is that the modifications made to the tpl_checkout_one_default.php file have rendered the page's HTML invalid.

Since much of the OPC's processing involves jQuery processing of the page, invalid HTML can confuse the data-gathering performed when that "Confirm" (or "Review") button is pressed.

I'd suggest updating the changes you've made to the template to restore its valid HTML; I'm pretty sure that with valid HTML you'll have the OPC working.