Issue
In rare cases, Drupal can show "A client error happened" inside content. The usual reason for this is because a browser, often Google Chrome, sends a Forwarded header. Symfony, an underlying library for Drupal, will display this message if it is confused by that header.
Resolution
To avoid this, make sure you have an up-to-date Drupal install, adding this code to your settings.php file (or post-settings-php hook for Site Factory customers):
unset($_SERVER["HTTP_FORWARDED"]);