To be able to connect ot icecast server set the password to /etc/icecast.xml as well to mixxx-> preferences to user: source password: hackme

<authentication>
        <!-- Sources log in with username 'source' -->
        <source-password>hackme</source-password> 
        <!-- Relays log in with username 'relay' -->
        <relay-user>relay</relay-user> 
        <relay-password>hackme</relay-password>
        <!-- Admin logs in with the username given below -->
        <admin-user>source</admin-user>
        <admin-password>hackme</admin-password>
</authentication>