Plesk for Windows login page shows: No connection could be made because the target machine actively refused it

Symptoms

1. Plesk login page is showing an error:

   PLESK_INFO: ERROR: PleskMainDBException
   No connection could be made because the target machine actively refused it.

   0: common_func.php3:637
   reconnect()
   1: common_func.php3:600
   db_connect()
   2: auth.php3:134

2. The following error appears:

   PLESK_INFO: [panel]: Error during product key mode determination, details: No connection could be made because the target machine actively refused it. ;
   trace:
   #0 C:Program Files (x86)ParallelsPleskadminplibdb.php(411): reconnect()
   #1 C:Program Files (x86)ParallelsPleskadminplibfunctions.php(2791): db_connect()
   #2 C:Program Files (x86)ParallelsPleskadminplibfunctions.php(2808): isPpaKeyRequired()
   #3 C:Program Files (x86)ParallelsPleskadminplibcommon_func.php3(7): require_once(‘C:Program File…’)
   #4 C:Program Files (x86)ParallelsPleskadminplibauth.php(93): require_once(‘C:Program File…’…