Web Server Rewrites: Apache to NGINX

I can't tell you what causes the problem you have because it seems to be quite localised. But here is the way to debug it.

The first line of your message leads us to Mage_Install_Model_Installer_Config::_checkUrl method. It sends request which is handled by Mage_Install_WizardController class.

Then the response body is checked in Zend_Http_Response::getBody if header contains transfer-encoding string and if it is chunked. If so Zend_Http_Response::decodeChunkedBody is called and if

! preg_match("/^([\da-fA-F]+)[^\r\n]*\r\n/sm", $body, $m)

The second line of your error is thrown.

So the first thing is to find out why do you have transfer-encoding=chunked and the second is why Mage_Install_Model_Installer_Config::_checkUrl is called at all as you said you disabled base url checks(s!).

UPDATE

Of course the easiest workaround would be to comment out the contents of Mage_Install_Model_Installer_Config::_checkUrl except return statement. Then you will be able to see if there will be more problems.

@handler is missing in your config. Try the following config. Update root and fastcgi_pass according to your environment.

server {   
    listen      x.xxx.xxx.x:80;
    server_name domain.es www.domain.es;

    root /home/user/web/domain.es/public_html;
    index index.php;
    location = /js/index.php/x.js {
        rewrite ^(.*\.php)/ $1 last;
    }

    ## Main Magento @location
    location / {
       try_files $uri $uri/ /index.php?$args;
    }


    ## These locations are protected
    location ~ /(app|var|downloader|includes|pkginfo)/ {
        deny all;
    }

    ## Images
    location ~* \.(js|css|png|jpg|jpeg|gif|ico)$ {
        expires max;
        log_not_found off;
        access_log off;
        add_header ETag "";
    }

    ## Fonts
    location ~* \.(swf|eot|ttf|otf|woff|woff2)$ {
        expires max;
        log_not_found off;
        access_log off;
        add_header ETag "";
        add_header Access-Control-Allow-Origin "*";
        add_header Cache-Control "public";
    }


    ## Execute PHP scripts
    location ~ \.php$ {
        try_files $uri =404;
        fastcgi_pass    127.0.0.1:9002;
        fastcgi_buffers 1024 4k;

        fastcgi_param  PHP_FLAG  "session.auto_start=off \n suhosin.session.cryptua=off";
        fastcgi_param  PHP_VALUE "memory_limit=768M \n max_execution_time=600";
        fastcgi_read_timeout 600s;
        fastcgi_connect_timeout 600s;

        fastcgi_index  index.php;
        fastcgi_param  SCRIPT_FILENAME  $document_root$fastcgi_script_name;
        include        fastcgi_params;

    }

}