I also found this problem in the creation of my site (WordPress 3.5.2 + Buddypress 1.7.2 + bbPress 2.3.2, default BP Theme, no other plugins, fresh installed…).
I was getting a Error 500 when antempting to log in.
Followed the instructions and now is solved, thanks!
Someone should include this bug into development for next version of BuddyPress!!
Thanks, however I do not have any other modification;
1. .htaccess is the original excluding the changes for the Network Install.
2. Other plugings beside BuddyPress and bbPress are deactivated. Even tried without bbPress and the efect is the same.
3. No custom code.
I saw that if I delete the cookies regarding the website, visiting it will not produce this efect, but once loged in and out, visiting the site will take you to the register page.
Still very anoying..