Hi Gary.
It sounds like something weird is going on, huh? :)
I don’t know what could be causing the trouble with the login or registration. s2Member doesn’t block login, but it can block registration if you disabled Open Registrations. [hilite path]Dashboard -› s2Member® -› General Options -› Open Registration[/hilite] But if you’re not allowed, there’d be a message explaining that registrations are closed, not just no page.
Do you have server logs that could help you see if there are errors related to that page? [hilite mono]wp-login.php[/hilite] is the one that is used for login and registrations.
It could be that one of the plugins you have installed is causing you this trouble. Try deactivating them one by one and test after each to see if you still get the problem.
About the PayPal thing with the user not being registered. Paying over at PayPal won’t automatically register the user in your site, he has to register his account after the paymet. But, you have the registration problem going on, so I’d try to fix that first.
I hope that helps. :)