The Rivendell Serial Number registry is no longer supported.
For various reasons we couldn't keep up with it, and it wasn't useful to anyone anyhow. So it's gone.
To actually register your bike in a useful way, register it with your city's police department and take a pic of the BB shell and send it to your insurance company.
If you want to know the history of your used Rivendell frame: ASK THE PERSON YOU BOUGHT IT FROM. Seriously. They know more than us, and a serial number doesn't tell as much as you'd think. We never filed them in the first place, and our best attempt at a registry was a noble, but ultimate failure. Thanks for your understanding and sorry for the hassle.