If you see the errors like the following:

2017/05/03 13:27:41 [crit] 4202#0: *1 connect() to unix:/var/run/php5-fpm.sock failed (13: Permission denied) while connecting to upstream, client: xx.xxx.xx.xx, server: localhost, request: "GET / HTTP/1.1", upstream: "fastcgi://unix:/var/run/php5-fpm.sock:", host: "xx.xx.xx.xx"

You can fix that issue by updating /etc/php5/fpm/pool.d/www.conf and set the listen.mode to 666 (remember to uncomment the line as well):

listen.mode = 0666