I'm just following up - I posted this bug a while ago, and when I didn't hear back, I researched and found the solution. (wahoo!)
PROBLEM:
If private-forums plugin is installed, users below moderator capabilities will have a BLANK Pick a Section dropdown on post-form.php. If they go ahead and try to click "submit" on their post, they get an error: Forum topic is closed. (error is bogus, obviously, because they are trying to post a NEW topic).
SOLUTION
In the /my-plugins/private-forums.php, replace line 294:
$new_forums[] = $forum;
with:
$new_forums[$forum->forum_id] = $forum;
WHY IT WORKS
The $new_forums array replaces the global $forums array with just the forums that user is permitted to see. The $forums array uses the forum_id as the array index. Without this fix, the $forums array will have the default index, i.e. 0, 1, 2, 3...
If your forum ids are (a) ordered numerically by their id and (b) no numbers are 'skipped' in the ids, you may not notice this issue. However, in my case, my forum ids are in the order "6, 7, 1, 10, 9" because we have added and deleted and reordered them over the life of the forum.
Hopefully this helps someone.
Cheers,
k