(Sorry, meant to add another post, ended up edited this one, I swear, I think I have a talent)
wp-signup.php and /register default to the home page.
No way for anybody to sign up. Yep, I deactivated plugins. And it's set to enable registrations on the site admin options.
This is across all themes, including the wpmu dev buddypress ones.
It's probably something dumb. I'd kill for a support system ticket set up for these questions.