NodeJS + MongoDB + Redis: Register and login - sessions issue

I've got a rather strange problem with my code on production server. On my MacOS it works perfectly, but when I deploy my app, I cannot login. After debug, I've found that I cant load session from req-object. Here's the code of all main parts (settings,, login page and main page after login)

//SETTINGS
var express = require('express');
var app = express.createServer();
var mongo = require('mongodb'),
    Server = mongo.Server,
    Db = mongo.Db,
    ObjectID = require('mongodb').ObjectID;
var BSON = require('mongodb').BSONPure;

var RedisStore = require('connect-redis')(express);

//connecting to mongo
var server = new Server('localhost', 27017, {
    auto_reconnect: true
});
var db = new Db('metadocs-node_db', server);

//setting express app
app.set('view engine', 'ejs');
app.configure(function () {
    app.use("/static", express.static(__dirname + '/static'));
    app.use(express.bodyParser());
    app.use(express.cookieParser());
    app.use(express.session({
        secret: "meta-meta",
        store: new RedisStore
    }));
    app.use(express.methodOverride());
});

//login page - POST for /login page
app.post('/login', function (req, res) {
    db.collection("users", function (err, collection) {
        collection.findOne({
            username: req.body.username
        }, function (err, doc) {
            if (doc && doc.password == req.body.password) {
                req.session.user_id = doc._id;
                res.redirect('/');
            } else {
                res.render('login.ejs', {
                    success_login: 1
                });
            }
        });
    });
});

//GET INDEX PAGE - only after login
app.get('/', loadUser, function (req, res) {
    db.collection("companies", function (err, collection) {
        collection.count(function (err, count) {
            res.render('index.ejs', {
                total_companies: count,
                current_user: req.currentUser['username']
            });
        });
    });
});

//loadUser() is function that creates/loads user session if possible
function loadUser(req, res, next) {
    if (req.session.user_id) {
        db.collection("users", function (err, collection) {
            collection.findOne({
                _id: new ObjectID(req.session.user_id)
            }, function (err, user) {
                if (user) {
                    req.currentUser = user;
                    next();
                } else {
                    res.redirect('/login');
                }
            });
        });
    } else {
        res.redirect('/login');
    }
}

Here's the problem code line:

if (req.session.user_id) {

in loadUser() function. The problem is req.session.user_id is empty – I've found it out while debugging every line step-by-step. What I'm doing wrong? It works on my Mac, but does't work on Ubuntu.