I think I found the solution and solved this problem. It was BP bug maybe?
It actually similar what @tagtomat mention above.
The steps are quite simple:
1. Go to a group page as Admin
2. Go to group Admin’s link
3. Then “Settings” link
4. Just click “Save Changes” button while the “Enable discussion forum” should be checked.
It seems that previous checked status on “Enable discussion forum” field doesn’t reflect to database or configuration. Hence we should “commit” this setting to confirm and activate the discussion forum feature. I have tested this for about our 40++ new forum setting and it works! Hope it helps!
