Which browser are you using when you find it? Are you finding the error at ALL sites or only one?
If you're seeing the error while attempting to visit only one site, then it's a site server problem of which you can't fix. Generally, the server doesn't exist.
Hope this helps.
I get the following message:
[[Host Address Not Found: The proxy could not find an IP address for the host in your request. Possible causes include mistyped URL, misconfigured DNS and transient network problems.]]
Is it server problem?