Right, just had this 'fun' myself and found a 'fix'.

a) restart your router/modem/other item that connects you to the net.
2) open a command prompt and type in "ipconfig /flushdns" then "ipconfig /release" and finally "ipconfig /renew".
iii) Try your client again.

This cleared my error and I was able to get back in game. Hope that helps.

Oh and for various values of useful; There's a thread in 'Accepted Bugs' concerning this topic.

EDIT: Huh, has anyone noticed this 'bug' seems to only crop up every 7 days?