Je n'arrive pas à comprendre le problème de configuration de nginx

Ce que tu as posté est vraiment désordonné. Nginx traite les directives de localisation dans un ordre spécifique, et n'en fait correspondre qu'une seule (sauf si elle est réécrite). Arrêtez de fumer un peu, effacez ce que vous avez, et essayez ceci :

server {
    listen       80;
    server_name  lc.wp.com;
    index index.php index.html;
    access_log /usr/local/var/log/nginx/lc.wp.access.log;
    error_log /usr/local/var/log/nginx/lc.wp.error.log debug;

    root /usr/local/www/playground/php/wordpress/;

    location / {
        try_files $uri $uri/ @wordpress;
    }

    location @wordpress {
        rewrite ^/([_0-9a-zA-Z-]+/)?files/(.+) /wp-includes/ms-files.php?file=$2 last;
        rewrite ^/([_0-9a-zA-Z-]+/)?wp-admin$ /$1wp-admin/ permanent;
        rewrite ^/[_0-9a-zA-Z-]+/(wp-(content|admin|includes).*) /$1 last;
        rewrite ^/[_0-9a-zA-Z-]+/(.*\.php)?$ /$1 last;
        rewrite .* /index.php last;
    }

    location ~ \.php$ {
        if (!-e $request_filename) {
            rewrite ^/[_0-9a-zA-Z-]+/(wp-(content|admin|includes).*) /$1 last;
            rewrite ^/[_0-9a-zA-Z-]+/(.*\.php)?$ /$1 last;
            rewrite .* /index.php last;
            return 404;
        }
        fastcgi_index index.php;
        fastcgi_intercept_errors on;
        fastcgi_split_path_info ^(.+\.php)(.*)$;

        fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
        fastcgi_param PATH_INFO $fastcgi_path_info;
        fastcgi_pass 127.0.0.1:9000;

        fastcgi_param  QUERY_STRING       $query_string;
        fastcgi_param  REQUEST_METHOD     $request_method;
        fastcgi_param  CONTENT_TYPE       $content_type;
        fastcgi_param  CONTENT_LENGTH     $content_length;

        fastcgi_param  REQUEST_URI        $request_uri;
        fastcgi_param  DOCUMENT_URI       $document_uri;
        fastcgi_param  DOCUMENT_ROOT      $document_root;
        fastcgi_param  SERVER_PROTOCOL    $server_protocol;

        fastcgi_param  GATEWAY_INTERFACE  CGI/1.1;
        fastcgi_param  SERVER_SOFTWARE    nginx/$nginx_version;

        fastcgi_param  REMOTE_ADDR        $remote_addr;
        fastcgi_param  REMOTE_PORT        $remote_port;
        fastcgi_param  SERVER_ADDR        $server_addr;
        fastcgi_param  SERVER_PORT        $server_port;
        fastcgi_param  SERVER_NAME        $server_name;
    }
}