Take a look into
WP Admin -> S2member -> PayPal® Options -> Account Details and see if ALL the fields are correctly filled. It's sometimes this that causes this problem.
This video may also be of interest:
http://www.s2member.com/paypal-options-buttons-video/Have you tried going over the
WP Admin -> s2Member -> PayPal® Buttons -> Shortcode Attributes ( Explained )?
Or if you are using pro-forms then go to
WP Admin -> s2Member -> PayPal® Pro Forms -> Shortcode Attributes ( Explained )It may help clear some things up, just check your button code compared to what's in this section.
Remember to create new buttons with the new info, just to make sure it works.
If all else fails then try changing the theme to the default Twenty Eleven theme (this one always works).
Try disabling all your plugins (don't include s2Member) and then enable them one by one until the problems appear again. You will need to do tests with each plugin you enable until the problem appears again.
Hope this helps.
Also remember to report back.
Otherwise you leave me worrying.