This MAY be a JavaScript error, not sure.
Could you try making a compatibility test?
Just disable each plugin one by one (without including s2Member) until the problem goes away.
Also could you please try changing the theme to the default one (TwentyEleven)? And see if it works after that.
You should try one at a time in order to make sure you find the correct problem.
Another fast way is to deactivate all the plugins (not s2Member) and change back to the default theme (TwentyEleven), and then clear your browser's cache. See if the problem goes away, if it does, then activate each plugin one per one until the problem appears again, then the theme you were using, that should show up the criminal.
If you find the criminal, could you please tell me which plugin/theme was causing the problems? Thanks.
Please tell me how it goes and I hope this helps.