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.

Hopefully soon.