I recently changed servers and applied all patches to my Magento 1.8 installation. After the migration everything seems to be working fine except for one thing, when customers cannot advance beyond selecting payment method and clicking continue. It stays on the same stage of the checkout process.
I checked things out with the browser debugger and there are three AJAX calls that are successful (200) when continue is clicked, but the UI doesn't advance to payment method. The 3 succesful AJAX calls I see upon clicking continue are: saveShippingMethod/ ?prevStep=shipping_method progress/