No modal followed by a Processing error

When I try to place an order, no modal window appears and I receive the error ‘There was an error processing your order. Please check for any charges in your payment method and review your order history before placing the order again.’ If I click ‘Place Order’ a second time it goes through to the ‘Thank you, your order has been received’ page with a payment status of pending, but the order is marked as cancelled and there’s no sign of a page to enter card details, even when I uncheck the option to open it in a modal window. This happens with versions 2.14.1 and 2.16.2. WooCommerce is at the latest version. I rolled back to version 2.9.1 to get it working again.