header("Location:") function doesn't work on live server

Question

I've developed my website on localhost, and I upload all the files on the server now. Unfortunately, when I go to the account page while I'm not connected, the redirection using the header('Location: /login') function isn't working. I've tried many solutions, like using the function ob_start() and ob_end_flush(), but it still doesn't work. I've also tried to change the paths, without any good results.

account.php

logged_only();
require 'includes/needed.php';

// ...

logged_only() function

function logged_only()
{
    if (!isset($_SESSION['auth'])) {
        header('Location: /login');
        exit();
    }
}

Hope you can help me