I kept running into this error on Plesk 11.5 while using NGINX and PHP-FPM. This particular check will work on Plesk and non-plesk systems alike. This post isn't a “solution” as much as it is a way to track down the root cause of the error.
[crit] 1605#0: *154 connect() to unix:/var/www/vhosts/system/curvve.com/php-fpm.sock failed (13: Permission denied) while connecting to upstream, client: ... upstream: "fastcgi://unix:/var/www/php-fpm.sock:"
Check if PHP-FPM is running
My output was “php-fpm dead but pid file exists”
service php-fpm status
service php-fpm restart
My output was as follows:
Starting php-fpm: [12-Jul-2013 22:12:11] ERROR: [pool www.domain.com] cannot get uid for user 'someuser' [12-Jul-2013 22:12:11] ERROR: FPM initialization failed
From there I just had to track down the domain error within Plesk. When I was done fixing the issue, I restarted PHP-FPM and NGINX and everything was golden.
I hope this helps! As always, feel free to comment if you have any questions.