1. Running secure pages on the Nginx web server. See http://drupal.org/node/390450#comment-1420518

The text from this reference below.

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If you are on Nginx, Then add the little code in your Nginx conf file.

fastcgi_param HTTPS on;

Here is all the code in my host conf at the php section.
  location ~ .*\.(php|php5)?$
  {
    #fastcgi_pass  unix:/tmp/php-cgi.sock;
    fastcgi_pass  127.0.0.1:9000;
    fastcgi_index index.php;
    include fcgi.conf;
    fastcgi_param HTTPS on;
  }

Reload your nginx server, then you may get the enable box active.
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