I've installed this plugin on two instances of WordPress. Each time, the Settings page fatal errored and I wasn't able to update the API key, password, etc.
The fix was to comment out Line 823:
return $this->callServer( $args );
return "Everything's Chimpy!"
I was able to access the Settings page again. Once I updated my Mailchimp settings, I reverted the fix and I didn't get the fatal error anymore.
If you do this, make sure to revert the fix too or you'll lose ping() functionality.