CRITICAL: Exception thrown when handling an exception

[rmxnnkoko]

i'm trying to connect my sub-domain to public folders .After 500 error, prod.log file shows :
request.CRITICAL: Exception thrown when handling an exception (PDOException: SQLSTATE[HY000] [2002] Connection refused at /var/www/vhosts/domain.fr/sub.domain.fr/vendor/symfony/http-foundation/Session/Storage/Handler/PdoSessionHandler.php line 460) {"exception":"[object] (PDOException(code: 2002): SQLSTATE[HY000] [2002] Connection refused at /var/www/vhosts/domain.fr/sub.domain.fr/vendor/symfony/http-foundation/Session/Storage/Handler/PdoSessionHandler.php:460)"} []