Lee la documentación (Traductor)
https://docs.joomla.org/Custom_error_pages
Ver esta parte:
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Joomla! uses the templates/system/error.php file to handle several HTTP Status errors, including "403 Forbidden", "404 Not Found", and "500 Internal Server" errors. You can style the error results, if desired.
It is important to understand that error.php is an independent file from the Joomla! CMS but dependent on the Joomla! Platform. Plugins do not run on the file.
You cannot include modules or use <jdoc:include> statements.
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Prueba realizar esto en un joomla limpio en localhost, si funciona en localhost y no en el sevidor, contacta con tu proveedor.