Hi Winson,
The error mentions that Dashboards Designer is unable to communicate with server (AVANT11) via port 6400.
Check whether you are able to Telnet to the server from your client machine. Open Run dialogue and enter telnet avant11 6400.
If you are not able to establish connection using Telnet, then it might be network or Firewall issue. If this is a network issue try creating a host entry in HOSTS file.
Regards,
Niraj.