ERROR: Unable to verify $_POST vars.
Please contact Support for assistance.

This is most likely related to an invalid configuration of s2Member, or a problem with server compatibility. If you are the site owner, and you're absolutely SURE that your configuration is valid, you may want to run some tests on your server, just to be sure $_POST variables are populated, and that your server is able to connect/communicate with your Payment Gateway over an HTTPS connection.

s2Member uses the WP_Http class for remote connections; which will try to use cURL first, and then fall back on the FOPEN method when cURL is not available. On a Windows server, you may have to disable your cURL extension; and instead, set allow_url_fopen = yes in your php.ini file. The cURL extension (usually) does NOT support SSL connections on a Windows server.

Please see this thread for details regarding the ideal server configuration for s2Member.