Office Task Pane app ajax call not working

Question

I'm working on this TaskPane app for office, and I would like to get some data from a php function that resides on our domain.

We're using the same php function in other apps, where I can just call it with some parameters passed to it in the URL, and it returns a simple answer.

Example: https://www.ourdomain.com/phpfunction.php?message=HELLO

The problem is, that I cannot seem to call this function via AJAX, from the TaskPane app, while debugging.

I added our domain to the App Domains in the application manifest, but it didn't help

Here is my AJAX call

function httpGET(theUrl, callback) {
  var dataToReturn;
  $.ajax({
    type: 'GET',
    url: theUrl,
    cache: false,
    async: true,
    data: "",
    success:  function(data, textStatus, result) {
        callback(data)
    },
    error: function (result, textStatus, errorThrown) {
        //ALLWAYS GOES HERE
    }
  });
};

I want to do it in pure javascript, so if it's possible, I don't want to use the custom .NET web service fix for this.

PS: I want to specify, that this exact same call is working perfectly from a mobile app or a browser based app. It's Office who's doing something here...