The PayPal Standard gateway shows that error when the API request set to PayPal to verify the payment fails. This could be an issue of cURL not working correctly on your host, which is needed to send HTTP requests. You also need OpenSSL as it'll send an HTTPS request in most cases. Specifically your CBSubs > History should have an error like the following logged.
"Paypal: Fraud attempt or Paypal value mismatch detected by Paypal: Error: return information didn't validate with paypal. Please tell sysadmin to enable IPN and PDT in his Paypal account."
This could also mean the payment amount was changed at PayPal and send back to CBSubs, which is a mismatch. Are you adding a discount to the purchase at PayPal instead of at CBSubs? Any amount greater than the CBSubs price is acceptable, but anything less than the CBSubs price it'll reject.