webpack-dev-server: Hot update not update the view (screen)

I wrote a little react app and serve static files from a separate express server (to redirect all sub-pathes to index.html [react-router]).

static server on port 8080 webpack hot dev server on port 8081

Browser console log:

[HMR] Waiting for update signal from WDS...
client:14 [WDS] Hot Module Replacement enabled.

after modify and save a file (e.g. router.js)

client:18 [WDS] App updated. Recompiling...
client:60 [WDS] App hot update...
dev-server.js:54 [HMR] Checking for updates on the server...
dev-server.js:34 [HMR] Updated modules:
dev-server.js:36 [HMR]  - 11
dev-server.js:40 [HMR] App is up to date.

But on the screen nothing change

The cli console log show:

Hash: a00a156bb0c50980187c
Version: webpack 1.5.3
Time: 483ms
                               Asset     Size  Chunks             Chunk Names
                              app.js   130429       0  [emitted]  app
                           vendor.js  1854993       1  [emitted]  vendor
0.919fa51221ca3cdacfb2.hot-update.js     3980       0  [emitted]  app
919fa51221ca3cdacfb2.hot-update.json       36          [emitted]  
chunk    {0} app.js, 0.919fa51221ca3cdacfb2.hot-update.js (app) 117673 {1} [rendered]
    [0] multi app 52 {0}
    [1] (webpack)-dev-server/client?http://localhost:8081 1674 {0}
    [2] ./src/app.js 2597 {0} [1 error]
    [4] (webpack)/hot/dev-server.js 2916 {0}
   [11] ./src/router.js 3370 {0} [built]
   [34] (webpack)-dev-server/client/web_modules/socket.io/index.js 1149 {0}
   [89] (webpack)-dev-server/client/web_modules/socket.io/socket.io.js 105915 {0}
chunk    {1} vendor.js (vendor) 1601853 [rendered]
    [0] multi vendor 76 {1}
.
.
.
  [257] ./~/react/lib/toArray.js 3176 {1} [built]
webpack: bundle is now VALID.

Structure

|- build/
|- dev/
   |- webpack.base.js
   |- dev-server.js
|- src/
   |- app.js
   |- router.js
|- static/
   |- index.html
   |- assets/
      |- ...some files...

package.json

  "dependencies": {
    "react": "^0.12.2",
    "react-router": "^0.11.4"
  },
  "devDependencies": {
    "express": "^4.11.1",
    "jsx-loader": "^0.12.2",
    "react-hot-loader": "^1.1.1",
    "webpack": "^1.5.3",
    "webpack-dev-server": "^1.7.0"
  }

webpack.base.js

var webpack = require('webpack');
var path = require('path');

var vendorLibs = [
    'react',
    'react-router'
];

module.exports = function(options) {
    return {
        context: path.join(__dirname, '../', 'src'),
        entry: {
            app   : [
                'webpack-dev-server/client?http://localhost:8081',
                'webpack/hot/dev-server',
                './app.js'
            ],
            vendor: vendorLibs,
        },
        output: {
            path             : path.join(__dirname, '../', 'build'),
            filename         : '[name].js',
            chunkFilename    : '[id].bundle.js',
            sourceMapFilename: '[file].map',
            pathinfo         : true,
            publicPath       : 'http://localhost:8081/'
        },
        module: {
            loaders: [
                { test: /\.js$/, loaders: ['react-hot', 'jsx?harmony'] }
            ]
        },
        resolve: {
            extensions: ['', '.js']
        },
        debug: options.debug,
        devtool: options.devtool,
        plugins: [
            new webpack.optimize.CommonsChunkPlugin({ name: 'vendor', minChunks: Infinity }),
            new webpack.HotModuleReplacementPlugin()
        ]
    }
};

dev-server-js

var webpack = require('webpack');
var WebpackDevServer = require('webpack-dev-server');
var express = require('express');
var path = require('path');
var fs = require('fs');

var app = express();
var config = require('./webpack.base.js')

// Redirect all non existing files to index.html
app.get('/*', function(req, res) {
    var filename = path.join(__dirname, '../', 'static', req.url);
    if (fs.existsSync(filename)) {
        console.log('static: ' + req.url);
        res.sendFile(filename);
    } else {
        console.log('static: index.html (' + req.url + ')');
        res.sendFile(path.join(__dirname, '../', 'static') + '/index.html');
    }
});

var compiler = webpack(config({
    devServer    : true,
    devtool      : 'eval',
    debug        : true
}));

var server = new WebpackDevServer(compiler, {
    contentBase: 'http://localhost:8081',
    hot: true,
    quiet: false,
    noInfo: false,
    lazy: false,
    watchDelay: 300,
    publicPath: 'http://localhost:8081/',
    stats: { colors: true },
});

server.listen(8081, 'localhost', function() {});
app.listen(8080);

app.js

<!DOCTYPE html>
<html>
<body>
    <div id="app"></div>
    <script src="http://localhost:8081/vendor.js"></script>
    <script src="http://localhost:8081/app.js"></script>
</body>
</html>

Is it a problem when serving the index.html from an other port? The docs say it is possible (http://webpack.github.io/docs/webpack-dev-server.html#combining-with-an-existing-server)