Not Found when using activity stream as front page with BuddyPress

I figured out how to make it work, but it’s a bit hacky. Essentially, I coped the index.php from activity into the the main index.php of my Thematic child theme, adding the Thematic actions afterwards. Same error persists if I leave it on “Activity Stream” for the front page, but when I return it to the default “Most recent posts”, my modified index.php in the root takes over. I’ll post my code in a few days if nobody answers my question with how I can get it to work through that option — I’m building this as a high-end in-house repeated use theme, rather not resort to what I did.

(Redid as answer since no response.)