Nginx – No write permission for WordPress under Nginx in CentOS 7

I have a CentOS 7 machine with Nginx running and the user is "nginx" (tghe default one in CentOS). I've installed WordPress, which is working perfectly, and I've give wrtite permissions for nginx user in the wordpress folder and subfolders (775), but a still get a permission error:

Unable to create directory wp-content/uploads/2015/05

I double checked which user is running nginx worker processes and it is nginx.

It must be something trivial that I missed. Can someone help me?

EDIT:

I noticed this in /var/log/nginx/error.log:

2015/05/03 20:42:41 [error] 23652#0: *1 FastCGI sent in stderr: "Unable to open primary script: /usr/share/nginx/html/index.php (No such file or directory)" while reading response header from upstream, client: XXX.XXX.XXX.A, server: XXX.XXX.XXX.B, request: "GET / HTTP/1.1", upstream: "fastcgi://unix:/var/run/php-fpm/php-fpm.sock:", host: "XXX.XXX.XXX.B"

2015/05/03 20:44:18 [error] 23652#0: *3 FastCGI sent in stderr: "Unable to open primary script: /usr/share/nginx/html/wp-admin/admin-ajax.php (No such file or directory)" while reading response header from upstream, client: XXX.XXX.XXX.A, server: XXX.XXX.XXX.B, request: "POST /wp-admin/admin-ajax.php HTTP/1.1", upstream: "fastcgi://unix:/var/run/php-fpm/php-fpm.sock:", host: "XXX.XXX.XXX.B", referrer: "http://XXX.XXX.XXX.B/wp-admin/"

2015/05/03 21:02:54 [emerg] 23803#0: getpwnam("http") failed in /etc/nginx/nginx.conf:6

EDIT 2

$ ps -elf|grep nginx
1 S root      2045     1  0  80   0 - 27393 sigsus 15:46 ?        00:00:00 nginx: master process /usr/sbin/nginx
5 S nginx     2049  2045  0  80   0 - 27503 ep_pol 15:46 ?        00:00:00 nginx: worker process
5 S nginx     2050  2045  0  80   0 - 27503 ep_pol 15:46 ?        00:00:00 nginx: worker process
5 S nginx     2051  2045  0  80   0 - 27503 ep_pol 15:46 ?        00:00:00 nginx: worker process
5 S nginx     2052  2045  0  80   0 - 27503 ep_pol 15:46 ?        00:00:00 nginx: worker process

The server configuration file is:

server {
    listen       80;
    server_name  XXX.XXX.XXX.B;

    location / {
        root   /var/www/wordpress;
        index index.php index.html index.htm;
        try_files $uri $uri/ /index.php?q=$uri&$args;
    }

    error_page 404 /404.html;
    location = /404.html {
        root /usr/share/nginx/html;
    }
    error_page 500 502 503 504 /50x.html;
    location = /50x.html {
        root /usr/share/nginx/html;
    }

    location ~ \.php$ {
        root /var/www/wordpress;
        fastcgi_pass unix:/var/run/php-fpm/php-fpm.sock;
        fastcgi_index index.php;
        fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
        include fastcgi_params;
    }
}

As for the nginx configuration file:

user  nginx;
worker_processes  4;

error_log  /var/log/nginx/error.log;

pid        /run/nginx.pid;


events {
    worker_connections  1024;
}


http {
    include       /etc/nginx/mime.types;
    default_type  application/octet-stream;

    log_format  main  '$remote_addr - $remote_user [$time_local] "$request" '
                  '$status $body_bytes_sent "$http_referer" '
                  '"$http_user_agent" "$http_x_forwarded_for"';

    access_log  /var/log/nginx/access.log  main;

    sendfile        on;

    keepalive_timeout  65;

    index   index.html index.htm;

    include /etc/nginx/conf.d/*.conf;

    server {
    listen       80 default_server;
    server_name  localhost;
    root         /usr/share/nginx/html;

    include /etc/nginx/default.d/*.conf;

    location / {
    }

    error_page  404              /404.html;
    location = /40x.html {
    }

    error_page   500 502 503 504  /50x.html;
    location = /50x.html {
    }
    }
}