The SPARQL-FED query runs fine against our live URIBurner Link from the Exploiting the LOD Cloud Knowledge Graph using SPARQL-FED post and against my local Virtuoso instance. So I am not sure why it would not work for you.
The Virtuoso 22023 Error The service {sparql-endpoint-url} has no description and the site is not responding as a SPARQL endpoint errors occur if the remote URL passed in the service clause is invalid ie not a SPARQL endpoint or the site is down.
But the site is up and running from my testing thus I would suggest trying again.