Seems to have been the discourse-akismet plugin. When I commented that out of app.yml and rebuilt my forum came back.
@eviltrout : I notice that you pushed a commit 6 days ago that has something to do with “rewiring” and that seems to have broken my forum.
I do want to keep akismet active though: does anyone know if there is a way to pull a specific revision of a plugin via app.yml?