Can't complete recovery (Error 451)

One of my employees needed a reset, so I started the recovery process...

He got the notification and did his part...

Then I got a notification that I needed to "Complete Recovery"

But now when I click to do so, I get an Error: Failed to complete recovery. Error code 451

There's no way to cancel and start over... so he's stuck and can't log in.

Actually, he now gets a message "Your recovery email is no longer valid. It may have been used already, or canceled by your account administrator."

But I didn't cancel it... and we didn't use it already... and I'm not sure why his recovery email would no longer be valid. 

Any ideas?