AngularJS POST request not returning a response when sessions are used

I am making a POST request in angular to an API that returns a JSON object. Everything was working perfectly until I added express.sessions to the mix. Now, the POST request still gives a 200 status, but doesn't return anything for 2 minutes (when it times out). The API gets called successfully and the behavior on the backend is all normal, but I can't retrieve the response from the server. In fact, it doesn't even call the .error() function. It just times out without doing anything.

The strangest part: when I click the link that makes the POST request a second time (immediately following the first), the request and response execute exactly as expected and nothing is wrong. The first request always times out.

When I remove express.sessions, everything works as expected as well.

I am using Express on Node, AngularJS on the frontend.

Here's the code for the POST request:

Request being made. This is within an Angular controller. (Behavior: Nothing gets logged. It doesn't reach success() or error() but times out after 2min):

  $http.post('/api/add_highlight', newHighlight).
    success(function(id){
      console.log('response from server: ' + id)
    }).
    error(function(id){
      console.log('server response failed')
    })

API (Behavior: Successfully saves entry into database. Console logs all show success and behavior is normal on the backend.):

exports.add_highlight = function(req, res) {
  console.log('API has received request to add highlight')
      var hl = new models.highlights ({
        start: req.body.start
        , end: req.body.end
        , contents: req.body.contents
      })
      console.log(hl._id)
      hl.save(function(err){
        if (!err) {
          console.log('successfully saved highlight to mongo.')
          res.json(hl._id)
        }
        res.send('failed')
      })
};

app.js (When the second line is commented out, everything behaves as expected):

  app.use(express.cookieParser());
  app.use(express.session({store: new RedisStore(), secret: '[...]'}));

Please let me know if you have encountered something similar before or have any ideas.

I highly suspect it's an issue with my usage of Angular, because another POST request from an example works perfectly fine. But I do suspect it also has something to do with how I'm supposed to work with sessions in Angular, because when I comment out the session line in app.js, everything works fine too.

Thanks!