[@robin-w Moderator 12 years ago]

sorry wp4.0 changed the way default permalinks worked, and this has broken bbpress child forums, which is I suspect your problem.

There is a trac ticket for this with WordPress.

The interim fix is to go to pretty permalinks (which you say you don’t want to) or only have forums at the top level. In theory this will be fixed in 4.0.1 whenever that comes out.