After trying to use the sandbox and constantly getting the following error:
php error message = fopen(https://svcs.sandbox.paypal.com/AdaptivePayments/Pay): failed to open stream: HTTP request failed! HTTP/1.0 400 Bad Request
I decided I'd switch to the Live environment. However, when I try to perform a Parallel Adaptive Payment (using a third party plugin for Magento), I always get the 550001 error (User is not allowed to perform this action).
According to what I read elsewhere, I've checked that my PayPal Business account is verified (though I haven't got a definitve proof of that - how would I do that?), that my credentials are correct (using this URL) and that my application included the Basic Payment permissions. Still not working.
What else should I check?