[Problem]: Call to undefined function ws_plugin__s2member_is_systematic_use_page

Thanks for reporting your experience with s2Member. Can you please post your question here? We’ll be happy to help you: http://www.primothemes.com/forums/viewforum.php?f=4

In the mean time, you might try re-installing the latest version again. It sounds like you still have some files in the /wp-content/plugins/s2member/ directory from your previous installation.