One of the reasons for this and nothing in the logs might be newer version of Nginx which and you will have to replace in your configuration

include fastcgi_params;

with

include fastcgi.conf;

Another problem is that you might need to add

fastcgi_param  SCRIPT_FILENAME    $document_root$fastcgi_script_name;

in /etc/nginx/fastcgi_params , might be called also /etc/nginx/fastcgi.conf
It can also be added where your php setting block in Nginx is.

Might expand the post in the future with other possible reasons.