websitesforanything on "[Plugin: MailChimp for WordPress] MailChimp API error when not logged in"

Additional info and testing.

The client has their website hosted on GoDaddy Managed WordPress (or whatever it's called - not straight Linux, but the actual WP config plan).

This includes a staging site, so I pushed the site to staging: http://d24.b72.myftpupload.com/homepage/

I then proceeded to test as follows. S=success, F=fail

Version 3.1.12-beta
STAGING SITE
LOGGED OUT
S,S,F,F,F,F
Switched to TwentySixteen theme
S,S,F,F,F,F
Deactiveated all other plugins (still TwentySixteen theme)
S,S,F,F,F,F
(at this point, I thought maybe it's a caching issue. The only variables remaining are the browser (always Chrome), the theme (TwentySixteen at this point), the plugin, the host, and MailChimp)
Flushed Cache via GoDaddy menu inside WordPress Admin, waiting 15 seconds
S,F,S,F,F,F,F,F

So I have eliminated plugin conflicts. I have eliminated theme conflicts. I am down to host, MailChimp, plugin, and browser.

VERY INTESTING!! never ONCE did it tell me that I was already subscribed, even though I used the same email address every time. Something is definitely wrong there. Also, the logs show almost 100% failures, even though on-screen I showed many successes! Hmm...

[2016-07-20 12:24:22] INFO: Form 732 > Successfully subscribed [redacted]@[redacted].com
[2016-07-20 12:44:51] INFO: Form 732 > Successfully subscribed [redacted]@[redacted].com
[2016-07-20 12:44:57] INFO: Form 732 > Successfully subscribed [redacted]@[redacted].com
[2016-07-20 12:45:04] INFO: Form 732 > Successfully subscribed [redacted]@[redacted].com
[2016-07-20 12:45:08] ERROR: Form 732 > MailChimp API error: 0
[2016-07-20 12:45:12] INFO: Form 732 > Successfully subscribed [redacted]@[redacted].com
[2016-07-20 12:45:15] ERROR: Form 732 > MailChimp API error: 0
[2016-07-20 12:45:59] ERROR: Form 732 > MailChimp API error: 0
[2016-07-20 12:46:07] ERROR: Form 732 > MailChimp API error: 0
[2016-07-20 12:47:10] ERROR: Form 732 > MailChimp API error: 0
[2016-07-20 12:47:13] ERROR: Form 732 > MailChimp API error: 0
[2016-07-20 12:47:23] ERROR: Form 732 > MailChimp API error: 0
[2016-07-20 12:47:26] ERROR: Form 732 > MailChimp API error: 0
[2016-07-20 12:48:20] ERROR: Form 732 > MailChimp API error: 0
[2016-07-20 12:48:36] ERROR: Form 732 > MailChimp API error: 0
[2016-07-20 12:48:44] ERROR: Form 732 > MailChimp API error: 0
[2016-07-20 12:49:33] ERROR: Form 732 > MailChimp API error: 0
[2016-07-20 12:50:00] ERROR: Form 732 > MailChimp API error: 0
[2016-07-20 12:50:18] ERROR: Form 732 > MailChimp API error: 0
[2016-07-20 12:50:33] ERROR: Form 732 > MailChimp API error: 0
[2016-07-20 12:50:39] ERROR: Form 732 > MailChimp API error: 0
[2016-07-20 12:50:44] ERROR: Form 732 > MailChimp API error: 0