Looks like a temporary internet glitch.
Reply from the Opera Forum member, Frenzie:
Sounds like the Apache 2 server at validator.w3.org had a problem when you tried to reach it, unless you’re still getting the same error right now.
Tried just now and it works well. Validator must be a busy, busy server.
Reply this time: “This document was successfully checked as HTML 4.01 Transitional!”