03:25 AM PST: Error – Unable to communicate with payment processor during checkout
“Merchants who were experiencing this issue were found to have changed the email address on record with PayPal. If you are seeing this error during checkout, please try deleting and recreating your PayPal Express settings in your Yahoo! Payment Manager.”
You can also follow this article http://help.yahoo.com/l/us/yahoo/smallbusiness/store/order/paypal/paypal-02.html
to enable Paypal APIs.