I cannot change the access permission for sub-forums or independent forums, that is I only can change permissions for a forum which have a category as parent. To check it do the following using the "advanced management" option in the "forum" category of the ACP menu:
1) create a "category 1" category,
2) create "forum 1" forum inside "category 1" with "private" set of permissions
3) create a "sub forum 1" inside "forum 1" with "private" permissions too
Now go to "groups"/"manage groups" and do the following:
4) create a "group 1" group
5) go to the "permissions" (simple mode) form for "group 1", change the access permissions for both "forum 1" and "sub forum 1" to "allowed access" and click submit
6) go again to the "permissions" form for "group 1" and you can see that only "forum 1" has retained the "allowed access" value while "sub forum 1" continues with "disallowed access"
Curiously this only happens if the parent forum ("forum 1" in this case") has been created by me. If I do all the above but instead of "forum 1" I use the "test forum 1" which is provided with the default IP installation (just changing it's permissions to "private") then the "sub forum 1" created inside it will accept the "allowed access" setting given to it in the step 5.
I reported this problem in the 1.2.0.27 bugs thread, but it passed along between all the others and I haven't got any clue on what can be causing the problem neither any confirmation of it by some other user.