It seems this problem only I have, but I don't even know where to dig...
There is one site, which is (possibly) from anywhere in the world besides my work.
On all sites I see the same thing about this error is that this is an April fool's joke. However, from any desktop PC, on any browser (unless you use proxies) and opens the same error. If it was the firewall, then surely the error would be different, not the http error.
But what else could it be???