Unexpected response code: 502 error when using socket.io with AppFog-Collection of common programming errors

When the user opens http://sync.aws.af.cm/, the Google Chrome JavaScript Console shows (within a few seconds):

Unexpected response code: 502 error

The app should remember the contents of the text area even after the user refreshes the page. The app works locally, but not when I upload it to AppFog, and I suspect the problem is the 502 error.

Do you know what might cause this error in this case?

What I’ve tried

I tried changing the script src= path in my client from /socket.io/socket.io.js to node_modules/socket.io/node_modules/socket.io-client/dist/socket.io.js as described here, but it did not help.

I also tried serving the app at Heroku instead of AppFog, but in that case I kept running into the issue described here even though I specified socket.io version 0.9.6.

I’ve also looked at the following questions, but the comments so far don’t seem to help me.

socket.io creating 502 Bad Gateway on connect / websocket invalid

Socket.IO node.js websocket connection invalid port 8081

socket.io with node.js not working as expected

Relevant server code

Here is the relevant server code:

var server = http.createServer(function (req, res) {
  ...
});

var io = require('socket.io').listen(server);

// Define events that can be triggered by the client
io.sockets.on('connection', function (socket) {
  socket.on('setServerVariable', function (value) {
    ...
  });
  ...
});

server.listen(process.env.VCAPP_APP_PORT || 3000);

Relevant client code

  window.onload = function () {
    ...
    var socket = io.connect(window.location.hostname);
    socket.emit('getServerVariable');
    ...
  };
...

$ af logs sync

====> /logs/stdout.log