I received the message you refer to in the help page on integrating e-junkie with third party services - I have no idea where to start to fix it. Are you able to tell where the process with Mailchimp is falling down?
This is a notification that for the transaction 6KN77216J31484121, we were unable to notify your server at http://eepurl.com/xxxxx as we we received the error: HTTP Error 302
POST /xxxxx HTTP/1.1
HTTP/1.1 100 Continue
HTTP/1.1 302 Moved Temporarily
Content-Type: text/html; charset=UTF-8
Date: Sat, 10 Sep 2016 14:12:05 GMT
Set-Cookie: AVESTAENVIRONMENT=prod; path=/