Some ppl lock their whole site down with S2Member so that if your not logged in you can't access anything, and when you go to their web address you see the login page.
Does your theme use the standard wordpress login or does it have a custom page?
When you go to
http://mydomain.com/wordpress/wp-login.php?action=register (after you substitute your website path in the link) does it show the page? or does that redirect to the home page as well?
Does temporarily deactivating your plugins make a difference?
Does temporarily changing to another theme, twentyten for example, make a difference?
Do you log out of wordpress before testing? and close and reopen your browser? do you have your browser set to automagically enter your admin login when you visit your site? Because according to here
http://buddypress.org/community/groups/how-to-and-troubleshooting/forum/topic/registration-link-redirects-to-home-page/ I think you will find that it only works for logged out users… after all, why should a logged in member want to register?
Is your wordpress install in a subfolder? and is the redirection including the subfolder? So if your blog lives at
http://mydomain.com/wordpress/ is the redirect link coming back from paypal
http://mydomain.com/wordpress/?s2member_paypal_notify=1 or is it sending back the link without the folder like this
http://mydomain.com/?s2member_paypal_notify=1Are you mixing links? as in having some that are
http://mydomain.com/wordpress/ and some that have the www bit in them like this
http://www.mydomain.com/wordpress/Since reinstalling didn't make a difference, it's likely to be setting somewhere that's so close to being right it's easily overlooked. (something I've found from past experiences)
Cheers
Kym