I am getting this error message when I configure the Shibboleth plugin the first time on my test server.

Invalid get_string() identifier: 'shibboleth_instructions' or component 'auth'

    * line 6058 of /lib/moodlelib.php: call to debugging()

    * line 6613 of /lib/moodlelib.php: call to core_string_manager->get_string()

    * line 245 of /auth/shibboleth/auth.php: call to get_string()

    * line 27 of /admin/auth_config.php: call to auth_plugin_shibboleth->process_config()}}

Workaround: But once I added the following to this file, 'lang/en/auth.php', the error goes away.

$string['shibboleth_instructions'] = 'Some text here.';

To reproduce this issue after you tried the workaround, make sure you

• clear the cache by calling 'admin/purgecaches.php';
• reset the value of auth_instructions in table mdl_config_plugins to [[shibboleth_instructions]].
• remove this line $string['shibboleth_instructions'] = 'Some text here.'; from lang/en/auth.php.
  It took me a while to figure this out.