Yes - we are using GJ1.8b5, CB 1.2.2, Joomla 1.5.15, and Kunena 1.5.10. But the fact that all the forums that we create manually work fine and all the GJ forums give us this error, makes me think that there is something wrong with the way GJ is creating the forum.
I don't know if this helps (screenshot attached), but in the Kunena admin for the forum created by GJ, there is no Public Access set (although the main Kunena dashboard for that forum says Public Backend for both the Public Access and Admin Access).