dskallman wrote:After looking at it more closely with hosting support, it seems that it is tied to s2-ssl issue. The form works well without it and does not redirect. Any other ideas of what we can look at or why it isn't working?
I get the same result. When I go to this page: 
https://ynpn.org/sfba/join, I am redirected to: 
http://ynpn.org/sfba/sfba/join ( notice the additional /sfba/ there? ).
When s2Member performs an SSL redirection, it converts 
http:// to 
https://, and it will add 
?s2-ssl=yes onto the end of the URL. Since that's not happening here, I suspect your problem is rooted inside the 
/.htaccess file on your server, or even perhaps with another plugin that is attempting to rewrite and/or reformulate Permalinks on your site.
Let's start, if we may, with your 
.htaccess file please. Can you please post a copy of what's inside the 
/.htaccess file for your installation of WordPress?
In addition, I would temporarily disable one other plugin at a time to see if you can narrow this down to a plugin conflict. I understand that you need each of your plugins, but sometimes disabling them temporarily will help you narrow down a conflict so it can be resolved.