I use the payment form on Joomla 2.5. On the Payment Confirmation page (where the client reviews the data provided in the form) the "Back" button doesn't work. When I click on it I get a 404 Error (page not found).
Interestingly, the URL shown in the browser is correct (I can copy and paste it into another browser tab and it works).
That's strange. Could you please submit a support ticket sending me the link to the form, also administrator and FTP account of your site so that I can check (and fix) it for you.
