I live in Italy. My credit card is from a US bank. When my IP is Italian, the checkout process goes from the "confirm order page" back to the home page of the store. When I jump in a US based VPN, the checkout process works. You might want to inform adyen.com that something is broken with their checkout process.
I guess it's worth to sent this issue to Uber Support: http://support.uberent.com/open.php I sure they can check why exactly it's happen.
