About an hour ago, we started receiving requests from people who got the following error when performing a sign-up request using the MailChimp for WordPress plugin. As we dig in, we'll use this page to keep you posted.
16:22 UTC: We confirmed this to be an issue on MailChimp's side. It is not a plugin issue.
16:30 UTC: MailChimp is aware of the issue and currently digging in. The issue is not limited to just our plugin, other programming languages communicating with the MailChimp API are receiving the same error response.
17:02 UTC: It seems subscribers are still going through, which is great news. We are performing some more tests.
— Daniel Wright (@Hobonium) May 15, 2015
17:22 UTC: We've found that the issue only occurs when double opt-in is enabled. Until MailChimp finds a resolution, please disable double opt-in (temporarily) in your form settings.
17:50 UTC: MailChimp just provided us with some more detailed information on the error.
We currently have a ticket open with our developers to get this looked into ASAP. For a bit of background, this error is occurring with all API subscribe calls with double opt-in enabled. It also does occur with single opt-in calls which have the send_welcome parameter set to send out the final welcome email.
In short, please disable double opt-in but do not enable the "send welcome email" option, for now.
18:05 UTC: MailChimp just managed to fix the issue. We're good - please switch back to your original settings if you applied the temporary fix.