Arcane problem with NIST time server

Two days ago I noticed that my Airport Extreme stopped updating its internal clock. I have been using the NIST time server nist1-ny.WiTime.net when all of a sudden its logs indicated "no address for NTP server" and it made no further attempts to contact it. My Airport Express, located downstream and configured as a bridge, did not have the same problem.

So I went to the NIST site and determined that URL is no longer there, but a new one: nist1-ny.ustiming.org is. So I updated the Extreme to use that instead... same problem: "no address for NTP server nist1-ny.ustiming.org".

If I change the time server to one of Apple's preloaded ones, like time.apple.com, it works fine. I would suspect a problem with my DNS server (I use OpenDNS), except that my Express had been happily using the old URL nist.ny.WiTime.net all this time. I don't believe NIST changed the time server's actual address anyway, just its name. I since modified the Express to use the new URL nist1-ny.ustiming.org and it still works. Meanwhile, my Extreme can't seem to connect to the same address.

I have a spare Extreme that I can substitute for troubleshooting purposes, but I'm wondering if someone has any idea why this would have happened.