I just did a test, on the failure page it is outputting the session. the response from paypal is coming back as:
Security error Security header is not valid
This means that paypal is rejecting the API credentials. this can happen for a few reasons:
1) you have sandbox enabled but are using the live API Credentials or Vice Versa.
2) You have not accepted the billing agreement for the API Credentials
to check the api credentials, go to:
WebAssist -> eCart -> Payment Gateway Credentials
here you can toggle sandbox mode on or off and enter the api credentials to be used.
In some cases, I have seen paypal just generate bad API credentials, you may want to request new ones.