Paypal has just updated their payment gateway (at least the sandbox) which is converting all cancel_url and return_url to lowercase but not the notify_url.
e.g. return_url = http://www.abc.com/Abc/Xyz/  => http://www.abc.com/abc/xyz/

This seems to be a javascript issue on the payment completion page and only happens if you manually click on the link to return to your site; the automatic redirection seems to work. This is clearly a Paypal bug and we need this resolved ASAP.