NGINX login problem

I have a problem with login to my application.
I use nginx as load-balancer and mongrel cluster as application
servers. I have 2 servers and 8 mongrels per server.
When I try to login directly on mongrel, everything is ok. But if I
use to login nginx, login sometimes failed - login and password are
nil in admin_controller and authentication failed in this case.

Do you have some idea to find solution of my problem?

nginx.conf :

#user and group to run as
user yeti yeti;

worker_processes 4;

pid /var/run/;

events {
   worker_connections 8192;
   use epoll; # linux only!

http {
   include /usr/local/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;

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

  sendfile on;

  tcp_nopush on;
  tcp_nodelay on;

client_body_temp_path /tmp 1 2;

  gzip on;
  gzip_http_version 1.0;
  gzip_comp_level 2;
  gzip_proxied any;
  gzip_types text/plain text/css application/x-javascript
                  application/xml application/xml+rss text/javascript;

# nginx virtual host configuration file
# to be included by nginx.conf
# Load balance to mongrels
upstream mongrel_cluster {
   server server1:3000;
   server server1:3001;
   server server1:3007;

   server server2:3000;
   server server2:3001;
   server server2:3007;


# Begin virtual host configuration
server {
   # Familiar HTTP settings
   listen 443;

   ssl on;

   ssl_certificate /usr/local/nginx/certs/server.crt;

   ssl_certificate_key /usr/local/nginx/certs/server.key;

   server_name localhost;
   root /home/usr/app/current/public;
   access_log /var/log/nginx/localhost.access.log main;
   error_page 500 502 503 504 /500.html;
   client_max_body_size 50M;

# First rewrite rule for handling maintenance page
   if (-f $document_root/system/maintenance.html) {
      rewrite ^(.*)$ /system/maintenance.html last;

location / {

     proxy_set_header X-FORWARDED_PROTO https;

    index index.html index.htm;
    # Forward information about the client and host
    # Otherwise our Rails app wouldn't have access to it
    proxy_set_header X-Real-IP $remote_addr;
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_set_header Host $http_host;
    proxy_max_temp_file_size 0;
    # Directly serve static content
    location ~ ^/(images|javascripts|stylesheets)/ {
      expires 10y;
    if (-f $request_filename) {
    # Directly serve cached pages
    if (-f $request_filename.html) {
      rewrite (.*) $1.html break;
    # Otherwise let Mongrel handle the request
    if (!-f $request_filename) {
      proxy_pass http://mongrel_cluster;