Ispconfig3 php5-fpm Error 500 Internal server error

When trying to change a client website on a Ispconfig3 host from Fast-CGI to PHP-FPM the page just gave Error 500 Internal server error. Investigating the site error log showed lines like:

[Sat Oct 22 10:39:56 2016] [error] [client xx.xx.xx.xx] (2)No such file or directory: FastCGI: failed to connect to server "/var/www/clients/client6/web447/cgi-bin/php5-fcgi-*-80-domain.xx": connect() failed
[Sat Oct 22 10:39:56 2016] [error] [client xx.xx.xx.xx] FastCGI: incomplete headers (0 bytes) received from server "/var/www/clients/client6/web447/cgi-bin/php5-fcgi-*-80-domain.xx"

Checking the status of PHP-FPM by the command:

service php5-fpm status

gave the result “not running”.

Investigating the PHP-log file, /var/log/php5-fpm.log, displayed lines like:

[22-Oct-2016 10:34:53] ERROR: [pool web405] cannot get uid for user 'web405'
[22-Oct-2016 10:34:53] ERROR: FPM initialization failed

However, the site widh id 405 and user web405 did no longer exist on the server and has been left there by Ispconfig3 for unknown reasons.

The solution was to manually remove the file /etc/php5/fpm/pool.d/web405.conf file and then:

service php5-fpm restart
service apache2 restart