Dropbox PHP SDK not working on production server - php

Question

I am playing around with Dropbox PHP SDK and I have a wired problem. On my localhost environment it works like a charm, but on my server it does not. I have ssl and the dropbox redirect urls is: https://flaviuscojocariu.com/arhitecta/dropbox_finished.php
I don't understand where the problem can be, as I searched my entire code for http://localhost/ links and replaced everything. I get only the 500 error and I don't understand why.
Can someone advice?
If needed I can provide more code. This is the code which I use for auth.
<?php
session_start();
$_SESSION['user_id'] = 1;
require __DIR__.'/../vendor/autoload.php';
$dropboxKey = '***';
$dropboxSecret = '***';
$appName = '***';
$appInfo = new Dropbox\AppInfo($dropboxKey,$dropboxSecret);
// Store CSRF token
$csrfToken = new Dropbox\ArrayEntryStore($_SESSION, 'dropbox-auth-csrf-token');
// Define auth details
$webAuth = new Dropbox\WebAuth($appInfo, $appName, 'http://localhost/arhitecta/dropbox_finished.php', $csrfToken);
$db = new PDO('mysql:host=127.0.0.1;dbname=arhitecta;', 'root','');
//User datails
$user = $db->prepare("
SELECT * FROM users WHERE id = :user_id
");
$user->execute(['user_id' => $_SESSION['user_id']]);
$user = $user->fetchObject();