Site does not work on PHP-FPM on Plesk: 503 service unavailable

Symptoms

• Sites that use the same PHP-FPM version (for example, 7.2 are not working):

  503 service unavailable

• The following error appears in the domain's error log /var/www/vhosts/system/example.com/logs/error_log:

  [proxy_fcgi:error] [pid 15605:tid 140353550149376] [client 203.0.113.2:45388] AH01079: failed to make connection to backend: httpd-UDS
  [proxy:error] [pid 15605:tid 140353373902592] (2)No such file or directory: AH02454: FCGI: attempt to connect to Unix domain socket /var/www/vhosts/system/example.com/php-fpm.sock (*) failed

• Socket /var/www/vhosts/system/example.com/php-fpm.sock does not exist and switching PHP handler to another one at Plesk > Domains > example.com > PHP Settings recreates it, but switching PHP handler back removes it again

Cause

Stuck or terminated PHP-FPM service.

Resolution

1. Login to the server via SSH

2. Restart PHP-FPM service that is used by domains:

   # service plesk-phpXX-fpm restart

   Note: Change XX to the corresponding version (e.g. plesk-php72-fpm)

3. Recreate sockets by executing the command:

   # plesk bin php_settings -u