That is normally a sign that a non-proper disconnect did happen from our tunnel service (please do not ask me what could have caused that exactly as I have no idea, could have been a crash, internet problems, …?). Because of that does the service still mean that you are connected and so blocks your subdomain. As the one that you request is blocked does it instead assign a random one. I did now restart the service which should have freed up your subdomain again. So if you restart, it should work again fine.