Hope my reply will help :
Collectif Dont Panic replied to the topic [Resolved] Can't register new users on my BuddyPress site in the forum Installing BuddyPress 9 months, 3 weeks ago
Hi there !
I was exposed to the same issue today, with BP 1.7.2 and WP 3.5.2.
Solved by checking the php version of the site, the php version should be 5.4 ( use the function <? phpinfo(); ?> to check that.
Other thing, like have said @baskers, ensure you have only lower caps in your url.
Hope my reply would help someone else !
@collectif-dont-panic Not recently active