I've experienced this problem here and there as well, always with test groups.
The reason is that the forum_id is associated with the group in the groupmeta table, but the forum itself does not exist. Not sure why this is the case, but I can hypothesize that during some BP upgrade in the past (mostly likely the one corresponding to BP 1.0 -> BP 1.1) the BP-internal bbPress upgrade script decided not to copy over any forums that were empty, ie that didn't have any topics associated with them. Yet somehow the forum_id remains in the db.
The solution to the problem, on a group-by-group basis, is to remove the groupmeta field that stores the forum_id, and then to toggle the group discussion forum off and on. In particular, when you turn it back on, BP automatically creates the missing forum.
I have done this with your testing testing group. If the problem arises in the future with particular groups, please open a ticket specific to that group.