The cause for these errors could be either server permission issues or mistakes in your IPN script code. We generate error messages when we don't get an HTTP 200 success response for any reason, including script error 500, SSL handshake validation, network timeout, or server error 503.
To solve IPN issues, you will need to debug your IPN script code. You may also need your Network or Systems Administrator to review your server permissions and logs to determine what's causing the error.
Visit How do I troubleshoot Instant Payment Notification (IPN)? for additional resources.