Perhaps this is just a duplicate post, I saw one nearly identical to this last night that a ticket was opened for. Please make sure you have updated the ticket by logging into WebAssist.com.
For other users that may be experiencing a problem like this if you use the incorrect PayPal credentials for either live or sandbox transactions you can get an error like this. You must ensure that you use Sandbox test credentials for testing in the sandbox and your regular live credentials for testing live transactions with the regular API.