I am getting this error message when I configure the Shibboleth plugin the first time on my test server.
Workaround: But once I added the following to this file, 'lang/en/auth.php', the error goes away.
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.