Plesk can’t start PHP-fpm

If Plesk cannot start PHP-fpm or all PHP web pages of a certain PHP version show the following error message:

Service Unavailable

The server is temporarily unable to service your request due to maintenance downtime or capacity problems. Please try again later.   Restart all PHP versions. PHP must be stopped, all lockfiles deleted and PHP restarted. To do this, connect to the server via SSH and execute the following commands

service plesk-php56-fpm stop
service plesk-php55-fpm stop
service plesk-php54-fpm stop
service plesk-php70-fpm stop
service plesk-php71-fpm stop
service plesk-php72-fpm stop
service plesk-php73-fpm stop
service plesk-php74-fpm stop
service plesk-php80-fpm stop
service plesk-php81-fpm stop
find /var/www/vhosts/system/ -name ‘php-fpm.sock’ -exec rm -v {} \;
service plesk-php81-fpm start
service plesk-php80-fpm start
service plesk-php74-fpm start
service plesk-php73-fpm start
service plesk-php72-fpm start
service plesk-php71-fpm start
service plesk-php70-fpm start
service plesk-php56-fpm start
service plesk-php55-fpm start
service plesk-php54-fpm start

It is normal that only PHP versions that are used can start.