Fatal error when registering or updating users

When trying to register users, or when updating any of the user's info, I get this error:

Fatal error: Class 'Mailchimp_List_DoesNotExist' not found in (mysiteurl)/wp-content/plugins/mailchimp-sync/mailchimp-api/Mailchimp.php on line 345

I'm using Register Plus Redux plugin for registration with custom user fields. It was working fine before, although I hadn't tested in a while. I also noticed a handful of these in the error log, the first being on May 6:

[May 6, 2014 4:18 pm] [CODE:200] [EMAIL:] - Invalid MailChimp List ID: