The is_available() function in Jabber's notification method class is only checking if the template file for sending the notification via email in language/en/email is present. However, the template files for Jabber notifications are located in language/en/email/short.

In short, this is what currently happens:

if ($email_template_exists)

{

    send_jabber_notification();

}

If an extension supports sending its notifications via email but doesn't provide a template file for Jabber, phpBB still tries to send a Jabber notification and fails with a PHP Fatal error: Uncaught exception 'Twig_Error_Loader' with message 'Unable to find template' , see https://www.phpbb.com/community/viewtopic.php?f=461&t=2505726