Same issue here. The page just reloads without anything happening. Tried tuning on/off tax, but didn't make any difference. Any ideas how to solve?
Error code:
ERROR - 2017-08-09 20:33:57 --> PayPalExpress::setExpressCheckout Error -->20005: 10002: Security header is not valid
(I'm using my production credentials, should they be different in Sandbox mode?)