Hi nattheman,
When you added: define( 'YIKES_MAILCHIMP_NULL_DEBUG', 1 ); and tried again, what was the result?
You might also want to try toggling on debugging on the settings page and trying again, with debug on. Hopefully that will log some sort of error.
The YIKES_MAILCHIMP_NULL_DEBUG constant should give us the best response from the API thought.
Evan