The registration form at purpleair[dot]com/register says " Looking good: Device-Id and Associated Email are related…".
However, the registration submission fails with " Unknown device! Please check the “Device-Id (MAC)” and try again. NOTE: If you previously registered this sensor, you will need to use the same “Owner’s Email” as the first registration…"
On June 29 I finally got a response to my June 17 email to contact@purpleair.com. My problem was due to a bad MAC device-ID label on the unit. When I used the corrected ID provided in the email response, registration succeeded.
Using the wrong email address in the registration seems likely to be the most common trigger of registration failure. However, the response “Looking good:
Device-Id and Associated Email are related…” seemed to rule out that problem in my case.