BuddyPress.org

Not sure, but I believe this has something to do with Multisite.

I don’t believe you can use “wp-login.php?action=register” in Multisite, only “wp-signup.php”.

You might want to create your own signup page. Read the “Worth Mentioning” section here:
http://www.wptavern.com/what-to-do-or-consider-after-you-enable-multisite-in-wordpress#comments (scroll up a bit)