Receiving 500 server error on WP 4.6+

Here is the complete log :

PHP message: PHP Fatal error:  Uncaught Error: Call to a member function fetchAll() on boolean in /var/www/timesheet/wp/wp-content/plugins/external-db-auth-reloaded/external_db_auth.php:341
Stack trace:
#0 /var/www/timesheet/wp/wp-includes/plugin.php(602): external_db_auth_check_login(NULL, NULL)
#1 /var/www/timesheet/wp/wp-includes/user.php(58): do_action_ref_array('wp_authenticate', Array)
#2 /var/www/timesheet/wp/wp-login.php(791): wp_signon(Array, '')
#3 {main}
  thrown in /var/www/timesheet/wp/wp-content/plugins/external-db-auth-reloaded/external_db_auth.php on line 341" while reading response header from upstream, client: 148.177.129.210, server: help.mytimesheet.be, request: "GET /wp-login.php HTTP/1.1", upstream: "fastcgi://unix:/run/php/php7.0-fpm.sock:", host: "help.mytimesheet.be"