WHMCS Tags troubleshooting-article Troubleshoot and resolve issues with No nameservers are defined errors while registering or transfering domains. Troubleshoot and resolve Oops! Access Denied - Forbidden errors while logging in to WHMCS. Troubleshoot and resolve warnings about the OPcache PHP extension. Troubleshoot and resolve CURL Error: OpenSSL SSL_read errors while working with domains. Troubleshoot and resolve problems due to PayPal® Instant Payment Notification (IPN) failures. PayPal payment gateways may generate specific subscription log entries. After attempting to migrate to the new PayPal Payments payment gateway module from a previous PayPal® module, you are experiencing problems with subscriptions.