Unable to establish connection with session error 10030

We have been using Ghost 11.5.1 for the last 12 weeks and all has been going well.

Suddenly this week I cannot connect to any session, it times out with an "Unable to establish connection with session error 10030" message.
This is the same if I PXE boot from the ghost server or boot from USB.  

BUT if I enter the server address under "Discovery Methord" when specifying the session number it connects straight away.

Any ideas what might have changed or how I can fix this?

Thanks in advance.

Peter