Facebook SDK error during login attempt: No URL set

I'm getting the following error message

Facebook SDK returned an error: No URL set!

Fatal error: Uncaught exception 'Facebook\Exceptions\FacebookSDKException' with message 'No URL set!' in /membri/csslab/sdk/src/Facebook/HttpClients/FacebookCurlHttpClient.php:83

Stack trace:

0 /membri/csslab/sdk/src/Facebook/FacebookClient.php(216): Facebook\HttpClients\FacebookCurlHttpClient->send('https://graph.f...', 'GET', '', Array, 60)

1 /membri/csslab/sdk/src/Facebook/Authentication/OAuth2Client.php(277): Facebook\FacebookClient->sendRequest(Object(Facebook\FacebookRequest))

2 /membri/csslab/sdk/src/Facebook/Authentication/OAuth2Client.php(226): Facebook\Authentication\OAuth2Client->sendRequestWithClientParams('/oauth/access_t...', Array)

3 /membri/csslab/sdk/src/Facebook/Authentication/OAuth2Client.php(166): Facebook\Authentication\OAuth2Client->requestAnAccessToken(Array)

4 /membri/csslab/sdk/src/Facebook/Helpers/FacebookRedirectLoginHelper.php(255): Facebook\Authentication\OAuth2Client->getAccessTokenFromCode('AQAAf_xMnFr0C7i...', 'http://csslab.a...')

5 /membri/csslab/login-callback.php(30): Facebook\Hel in /membri/csslab/sdk/src/Facebook/HttpClients/FacebookCurlHttpClient.php on line 83

when trying to login with Facebook credentials.

Everything works fine in local. I have properly set the domain in Facebook App.

The code I'm using is that suggested by Facebook's documentation:

<?php // login-callback.php
session_start();

function redirect($url, $permanent = false) {
    if (headers_sent() === false) {
        header('Location: ' . $url, true, ($permanent === true) ? 301 : 302);
    }
    exit();
}

$path = '';
$uri = 'http://xxxxx.altervista.org';
require_once $path . 'sdk/src/Facebook/autoload.php';

$fb = new Facebook\Facebook([
    'app_id' => 'xxxxxxxxxxx',
    'app_secret' => 'xxxxxxxxxxxxxxxxxxxxxx',
    'default_graph_version' => 'v2.5',
    ]);

$helper = $fb->getRedirectLoginHelper();

try {

    $accessToken = $helper->getAccessToken();

} catch(Facebook\Exceptions\FacebookResponseException $e) {
    // When Graph returns an error
    echo 'Graph returned an error: ' . $e->getMessage();
    exit;
} catch(Facebook\Exceptions\FacebookSDKException $e) {
    // When validation fails or other local issues
    echo 'Facebook SDK returned an error: ' . $e->getMessage();
    exit;
}

if (isset($accessToken)) {
    // Logged in!
    $_SESSION['facebook_access_token'] = (string) $accessToken;

    // Now you can redirect to another page and use the
    // access token from $_SESSION['facebook_access_token']
    redirect($uri . 'survey.php?token='.$_SESSION['facebook_access_token'] ,   false);
} elseif ($helper->getError()) {
    // There was an error (user probably rejected the request)
    echo '<p>Error: ' . $helper->getError();
    echo '<p>Code: ' . $helper->getErrorCode();
    echo '<p>Reason: ' . $helper->getErrorReason();
    echo '<p>Description: ' . $helper->getErrorDescription();
    exit;
}
?>

UPDATE:

 public function send($url, $method, $body, array $headers, $timeOut)
{
$this->openConnection($url, $method, $body, $headers, $timeOut);
$this->sendRequest();

if ($curlErrorCode = $this->facebookCurl->errno()) {
    throw new FacebookSDKException($this->facebookCurl->error(), $curlErrorCode);
}

// Separate the raw headers from the raw body
list($rawHeaders, $rawBody) = $this->extractResponseHeadersAndBody();

$this->closeConnection();

return new GraphRawResponse($rawHeaders, $rawBody);
}