I think there may be an issue between s2member v 111029 and the mailchimp plugin v 1.2.5 by mail chimp and crowd favorite.
When theses plugins are installed I see two Settings options in the s2m plugin dashboard. One for s2m and the other goes to the mailchimp control panel.
Thanks for pointing that out now. It looks like the MailChimp developers did something wrong with the return-values for the settings. I would write in and tell them that.