We just shipped a brand new media player to the client's location - and the same error comes up on the screen - "Website not found". We've now asked the client to take a picture of the back of the media player, so we can show that the network light is showing. We also asked for a picture of the back of their router.
The next step is to escalate this issue to their network techs internally so they can log into the router and determine whether there's something blocking the media player from being able to access the internet.
This is a very common problem, wherein something has happened at the location which has changed the programming at the router level. We think in this case perhaps a recent power outage has made it so the networking rules which allow the media player unfettered port 80 internet access have changed.