BuddyPress.org

We need to narrow this down further. Make a copy of the custom theme you’ve created. Save it someplace. Wipe the directory /wp-content/themes/home. That’s where the theme labeled “WordPress mu Homepage” is located. I’m guessing you modified that one. Upload a fresh copy of that directory to the server. No mods nothing. Right out of the download zip. Activate that theme and then see if you get the error.