To resolve the following error message
Stopping php-fpm: [FAILED] Starting php-fpm: ERROR: An another FPM instance seems to already listen on /var/www/vhosts/system/example.com/php-fpm.sock ERROR: FPM initialization failed [FAILED]
Connect to the Plesk server via SSH.
Move configuration files of the domains mentioned in the error message from both locations:
service php-fpm start