I think I have everything configured correctly, but when I use encrypted button mode, the PayPal Sandbox will not accept it (generic error message "Message 3005"). This happens whether or not I configure my PayPal Website Payment Preferences to require encrypted buttons.
When I look at the source code of my page, I see that s2Member did, indeed, created what looks like an encrypted button (there is a PKCS#7 encrypted string in a hidden field). I'm crypto-savvy, but not familiar with PayPal's API, so I'm not sure how to diagnose further.
Any recommendations as to how I can get encrypted buttons to work? (Also note that everything works splendidly without encryption.)
UPDATE: Just got word from PayPal that encrypted buttons might not work in the Sandbox... seeking further confirmation from PayPal...