Not sure if your problem is solved, but I encountered the same problem, but think I found the cause of this problem.
After 3 hours of reinstalling, clearing cache etcetc I found out that Mailchimp at some point has changed their pricing plan. The free plan comes now with only 1 audience list. Whereas I was still running Mailchimp with 3 audience lists. When I deleted 2 lists and reconnected with the Mailchimp list that was left, after about 2 hours later the optin was working again.
Hope this is of help! Regards Rob
