PHP Error if username not found (white page)

Question

Whenever I enter the correct user data the login works, but so I enter a wrong nickname comes a white page.

I have tried everything but nothing worked.

The try also shows no error!

Is the if statement wrong?

<?php
    define('DB_SERVER', '');
    define('DB_USERNAME', '');
    define('DB_PASSWORD', '');
    define('DB_DATABASE', '');

    $db = mysqli_connect(DB_SERVER,DB_USERNAME,DB_PASSWORD,DB_DATABASE);

    if($_SERVER["REQUEST_METHOD"] == "POST") {
        try{
            $myusername = mysqli_real_escape_string($db,$_POST['login']);
            $mypassword = mysqli_real_escape_string($db,hash('ripemd160', $_POST['pass']));
            $sql = "SELECT username, password, active FROM User WHERE username = '$myusername' and password = '$mypassword' LIMIT 1";
            $result = mysqli_query($db, $sql);
            if(mysql_num_rows($result) !== 0)
            {

                while($row = mysqli_fetch_array($result,MYSQLI_ASSOC)) 
                {
                    $username = $row['username'];
                    $password = $row['password'];
                    $active = $row['active'];

                    if($username == $myusername){
                        if($active == 1)
                        {
                            session_start();
                            $_SESSION["login"] = $myusername;
                            echo $_SESSION["login"];

                            header("Location: http://www.example.de"); 
                        }
                        else
                        {
                            header("Location: http://www.example.de/Main/Home/Views/Login/PleaseVerify.php");
                        }
                    }
                    else 
                    {
                        header("Location: http://www.example.de/Main/Home/Views/Login/WrongPassword.php");
                    }
                }
            }
            else 
            {
                header("Location: http://www.example.de/Main/Home/Views/Login/WrongPassword.php");
            }
        }
        catch(Exception $e)
        {
            echo 'Exception abgefangen: ',  $e->getMessage(), "\n";
        }
    }
    ?>