The app doesn’t work if you have set – irrevocably, as we all remember – your custom login domain, in this case padp.my.salesforce.com
IF you try to auhhenticate from here, you get an error.
Whereas in a Sandbox, it works just fine. Annoying.
Admittedly, MailChimp is on it.
As you can see below, and they do clarify the situation perfectly, except in on eregard: timing for a fix. Ugh.