Redirect loop on appearance customize page only?

I have my wordpress install sitting behind a Nginx reverse proxy. I have it configured to sit on the /blog/ filepath rather than on root. So far everything is working good after playing with the configuration a little.

Specifically I have this in my wp-config file since my reverse proxy is using SSL:

define('FORCE_SSL_ADMIN', true);
if ($_SERVER['HTTP_X_FORWARDED_PROTO'] == 'https')
       $_SERVER['HTTPS']='on';

And to get around an odd permissions issue I had to modify a line of code in wp-includes/user.php:

if (is_blog_admin() || is_network_admin() || empty( $_COOKIE[LOGGED_IN_COOKIE] ) )

Needed to be changed to:

if ( is_network_admin() || empty( $_COOKIE[LOGGED_IN_COOKIE] ) )

And I use nginx instead of Apache, with the following configuration for hosting my wordpress backend:

server {
  listen 80;
  server_name example.com www.example.com;

  access_log   /var/log/nginx/wordpress.access.log;
  error_log    /var/log/nginx/wordpress.error.log;

  root /var/www/wordpress;
  index index.php;

  location @wp {
    rewrite ^/blog(.*) /blog/index.php?q=$1;
  }

  location ^~ /blog {
          alias /var/www/wordpress;
          index index.php;
          try_files $uri $uri/ @wp;

          location ~ \.php$ {
                  include fastcgi_params;
                  fastcgi_param SCRIPT_FILENAME $request_filename;
                  fastcgi_pass 127.0.0.1:9000;
          }
  }
}

The issue I’m running into is that whenever I try to visit the customize appearance page I seem to run into a redirect loop. I seem to be able to access all the other pages fine. Visiting that page from the admin panel sends me off to this address: https://www.example.com/blog/wp-admin/customize.php?return=%2Fblog%2Fwp-admin%2Fthemes.php