As result, Moodle present a logout behaviour different from usual user experience and in case of the current page is not accessible to anonymous user even an unexpected error.
Proposed solution: use the global $redirect variable to retain the usual Moodle behaviour.
After logout, plain Moodle redirects the user to either the login page or the site index (from Moodle login/logout.php):
This auth/saml plugin, instead, redirects the user to the current page (from Moodle SAML Auth plugin index.php):
As result, Moodle present a logout behaviour different from usual user experience and in case of the current page is not accessible to anonymous user even an unexpected error.
Proposed solution: use the global $redirect variable to retain the usual Moodle behaviour.