Maatma is responding with this error page because the server takes too many time to respond.
First, check that your tunnel is currently up on the Maatma's tunnels page, then try to ping your gateway (adress on the tunnels page).
If it's up, most probably your firewall is filtering the request with a DROP target and never answer that the port is closed. Or your server is overloaded and really takes too long to respond.