Unable to access example.com website on the particular PHP version: 503 Service Temporarily Unavailable

Symptoms

• Unable to access the example.com website on the particular PHP version:

  CONFIG_TEXT: 503 Service Temporarily Unavailable

• In an attempt to switch PHP version in Plesk > Domains > example.com > PHP Settings the process fails with the following error:

  PLESK_ERROR: Error: phpinimng failed: ERROR:fpm_util:check failed service plesk-phpXX-fpm socket /run/plesk/plesk-phpXX-fpm.sock reason [Errno 111] Connection refused: '/run/plesk/plesk-phpXX-fpm.sock' 
   

• It is not possible to start affected PHP service with the error below in /var/log/plesk-phpXX-fpm/error.log file:

  CONFIG_TEXT: ERROR: An another FPM instance seems to already listen on /var/www/vhosts/system/example.com/php-fpm.sock
  ERROR: FPM initialization failed `/var/www/vhosts/system/example.com/php-fpm.sock` failed

Cause

The /var/www/vhosts/system/example.com/php-fpm.sock file is corrupted.

Resolution

1. Log into the server via SSH.

2. Remove the /var/www/vhosts/system/example.com/php-fpm.sock file:

   # rm -rf /var/www/vhosts/system/example.com/php-fpm.sock

3. Log into Plesk.
4. Go to Domains > example.com > PHP settings.
5. Switch PHP version to another:
   
    
6. Scroll down and press the Apply button.
7. Switch PHP version back to the required.
8. Scroll down and press the OK button to apply the changes.