Fatal error detected by WordPress

Hello,
I got a warning automaticaly sent by my WordPress site saying that a fatal error occurs because of a plugin : “Authorizer”.
I’m not sure this plugin is the cause of this E_ERROR, please tell me if I should change something manually…
Here’s a part of the message (sorry, partly in french) :

Détails de l’erreur
======================
Une erreur de type E_ERROR a été causée dans la ligne 2138 du fichier /home/****/www/wp-content/plugins/authorizer/vendor/phpCAS-1.3.6/source/CAS/Client.php. Message d’erreur : Uncaught exception 'CAS_AuthenticationException' with message 'CAS URL: https://login.mysite.fr/samlValidate?TARGET=http%3A%2F%2Fmysite.fr%2Fwp-login.php%3Fexternal%3Dcas%26redirect_to%3Dhttp%253A%252F%252Fmysite.fr%252Fwp-admin%252F
Authentication failure: SA not validated
Reason: bad response from the CAS server
CAS response: le ticket 'ST-65182-PDTgfMMgBwWQUNYxgWHY-login' est inconnu

Last line translation:
CAS response: the ‘ST-65182-PDTgfMMgBwWQUNYxgWHY-login’ ticket is unknown.

Thanks in advance.