WHMCS Tags troubleshooting-article Troubleshoot and resolve TypeError guessFileExtension() errors. Troubleshoot and resolve Unable to communicate with the WHMCS installation errors. Troubleshoot and resolve an Unable to display TLD cost pricing error. Troubleshoot and resolve Call to undefined function curl_init() errors. Troubleshoot and resolve issues with customers receiving unexpected subscription cancellation emails from PayPal®. Troubleshoot and resolve The sender's email address is not registered. (WHMCS 8.4 and later) or Unregistered Email Address (WHMCS 8.3 and earlier) errors. Troubleshoot and resolve Whoops Exception Maximum execution time of 30 seconds exceeded errors.