ERROR [2] mysqli_connect(): (HY000/2002): Connection refused

  Fatal error in line 5 of file /var/www/vhosts/lidoaltanea.it/httpdocs/includes/mysql_lib.php, PHP 5.6.40-55+0~20211023.59+debian10~1.gbp650f36 (Linux)

This is the debug trace

Error: "mysqli_connect(): (HY000/2002): Connection refused" (error #2).
Backtrace:
  0: myErrorHandler 
  1: mysqli_connect (line 5 in /var/www/vhosts/lidoaltanea.it/httpdocs/includes/mysql_lib.php)
  2: db_connect (line 69 in /var/www/vhosts/lidoaltanea.it/httpdocs/includes/inc_conn.php)
  3: include_once (line 6 in /var/lib/security/mw_portali/scheda.php)
  4: include (line 1 in /var/www/vhosts/lidoaltanea.it/httpdocs/scheda.php)
Variables in myErrorHandler ():
Client IP: 18.222.35.77
Aborting...