Why does mydomain.com/registrations/ return a blank page on my WPMS/Buddypress site?

I'm running a multi-site network with Buddypress nw activated and bbPress activated on the main domain. Suddenly, my registrations page is coming up completely blank. It used to work with all the same plugins activated. I think it might have happened on one of the recent upgrades. I've tried deactivating Buddypress (which returns the mydomain.com/wp-signup.php page instead of the */register page. Deactivating other plugins have had no effect. I don't want to share the URL publicly but will provide it privately upon request. I've firebugged it but to no avail (but I'm a blogger, not a dev my code proficiency is weak). Can you help?