jQuery AJAX request error status 0 - ajax

Question

I've been developing an application locally and am running into trouble on the actual server.
Whenever I submit an AJAX request with jQuery it gives me an error with error status:0 and and statusText: 'error'.
The Chrome inspector doesn't even show a response code for the request, it just says failed.
When I inspect it closer, I notice that all of the data was sent and the PHP file actually processed it. For example, in the request below, the user was indeed created. The bad response code is preventing other requests from executing (since they depend on a 'successful' response).
Here is a sample request:
var cct = $.cookie('ttc_csrf_cookie'); // csrf protection
var sUrl = "<?= base_url(); ?>user/create/";
var serialized = {
name: me.name,
email: me.email,
oauth_provider: 'facebook',
oauth_uid: me.id,
ttc_csrf_token: cct
};
$.ajax({
url: sUrl,
type: "POST",
data: serialized,
error: function(someStuffHere) {
//* THIS CODE RUNS. SHOWS ERROR STATUS: 0 */
},
success: function(user_id) {
//******** THIS HERE NEVER RUNS ***********///
}
});
And here is the corresponding PHP code:
public function create() {
if($this->input->post()) {
$user['name'] = $this->input->post('name');
$user['email'] = $this->input->post('email');
$user['oauth_provider'] = $this->input->post('oauth_provider');
$user['oauth_uid'] = $this->input->post('oauth_uid');
$user['last_activity'] = date('Y-m-d H:i:s');
$user_id = $this->Users->create_user($user);
$this->session->set_userdata('user_id', $user_id);
echo $user_id;
}
}
These two snippets are only an example. All AJAX requests won't work on the live sever.
What could it possibly be? Thanks!
UPDATE: I've narrowed it down and the issue occurs when echo'ing a result. When I comment out the echo, no error is thrown (but, of course, no result is sent).

Answer 1

Turns out that the issue was caused by the compress_output option in CodeIgniter. When it's set to true, the echo's don't work. Thanks for everyone that tried to help!