Salve a tutti, ottengo questo errore quando uso la session_start()

FATAL [/var/www/vhosts/tricommerce.it/httpdocs/_mojavi/opt/user/SessionContainer.class.php:59] session_start(): open(/var/lib/php/session/sess_98b59bb7b5392a1394d11632d1928e7a, O_RDWR) failed: Permission denied (13)

La dir /tmp ha i permessi 776...sapete come posso risolvere?

grazie