Then could you try making a compatibility test with the other plugins and themes? Just to be able to rule that out.
Just disable each plugin one by one (without including s2Member) until the problem goes away. If you can't make the changes site-wide because you already have users or somethings like that, you can use the
Plugin Organizer plugin if you want in order to only disable the plugin on the page with the problem.
Also could you please try changing the theme to the default one (TwentyEleven)? And see if it works after that. Again, if you can't disable the theme in a site-wide manner you can use the
Page Theme plugin in order to change the theme to the default theme in a specific page.
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.