You probably got a misconfiguration in your nginx VHOST. Are you sure you got the following configuration?

server {
        listen 80 default_server;
        listen [::]:80 default_server ipv6only=on;

        root /var/www/html;
        index index.php index.html index.htm;

        server_name your_domain.com;

        location / {
                # try_files $uri $uri/ =404;
                try_files $uri $uri/ /index.php?q=$uri&$args;
        }

        error_page 404 /404.html;

        error_page 500 502 503 504 /50x.html;
        location = /50x.html {
                root /usr/share/nginx/html;
        }

        location ~ \.php$ {
                try_files $uri =404;
                fastcgi_split_path_info ^(.+\.php)(/.+)$;
                fastcgi_pass unix:/var/run/php5-fpm.sock;
                fastcgi_index index.php;
                include fastcgi_params;
        }
}

To check what is causing this error, check your nginx and php logs. This could provide you some more information