I fixed this,
Apparently the plugin Shibboleth Authentication Plugin hasn’t been installed properly and it crashed. I had to disable this plugin through the database and now the login system is working properly again.
Here is the query
UPDATE plugin_settings SET setting_value=‘0’ WHERE plugin_name=‘shibbolethauthplugin’ and context_id =‘0’ and setting_name =‘enabled’;
Hope this can be helpful for someone else, I have seen that this problem appears few times in the forum.
The update was from 3.0 to 3.1.1, to upgrade I followed the tutorial through FTP and then I ran the script php /tool/upgrade upgrade in order to update the database.
Thanks