Mailchimp integration fails

Hello WPMU team,
the last of my worries with Hustle was the oddest.
I ran the whole MailChimp API integration and added the lists and interest groups needed.
Even ran the first test and got the welcome mails and all.
Then it went crazy. Started giving out “Something went wrong. Please try again” upon pressing the “sign-up” button.
I can see from the debug log that the API calls are returning ok :
Call to https://us15.api.mailchimp.com/2.0/lists/subscribe.json: {“id”:”abc”,”email”:{“email”:”[email protected]”},”merge_vars”:{“groupings”:[{“id”:”1751″,”groups”:[“blog articles”]}]},”email_type”:”html”,”double_optin”:true,”update_existing”:true,”replace_interests”:true,”send_welcome”:false,”apikey”:”xxxx”}
[21-Apr-2017 13:49:46] Completed in 715.86ms
[21-Apr-2017 13:49:46] Got response: {“email”:”[email protected]”,”euid”:”abc”,”leid”:”def”}

Nonetheless i see the “admin-ajax.php” call as “stalled” and it goes red after a few seconds never returning “success”.
I’ve left it to CSV collection for now, but would really love to use MC for our newsletters.

Thanks again, looking forward to provide more info to resolve this.