My registration link is broken too. I’m using the latest versions of everything (WPMU), no active plugins except BuddyPress and WordPress.com Stats, no custom language, and the default theme. Any ideas?
I once wrote some code which adds a setting to the BuddyPress administration panel. It was probably the very first BuddyPress plugin I ever wrote and I should touch it up and put it in the repository. In the meantime, see it here: