@msdeadlines - It turns out I had the same permissions problem, but I hadn't noticed (didn't actually trying clicking on the Mailchimp Settings). Anyway, I found the fix to that one, too. The Mailchimp plugin is checking for user capability 'edit_plugins', which seems logical, but it turns out that the 'edit_plugins' cap can be switched off globally by the command:
define( 'DISALLOW_FILE_EDIT', true );
In my case, I am using the "Better WP Security Plugin" and have ticked the option to "Turn off file editor in WordPress Back-end." which means admins no longer have the 'edit_plugins' capability, and therefore the Mailchimp settings page doesn't let them in. You may have a similar security plugin installed, or even a theme that is disabling the file editor. The quick fix is to change the defined cap on line 31 of mailchimp.php:
I usually use 'manage_options' on my plugins. Once you're able to get to the settings page, you may just need to check that everything is configured correctly, and hopefully your form submission problem will get fixed. Based on the error you posted, it sounds like something to do with your Interest Groups settings.
For extra information about the 'edit_plugins' cap issue: