[@robin-w Moderator 12 years, 1 month ago]

Ok, I know what the problem is

for a forum structure where the parent is forum-a and the sub-forum is forum-b

under 4.0 the permalink is

http://www.mysite.com/?forum=forum-b

whereas to work it should be
http://www.mysite.com/?forum=forum-a/forum-b

So now we have a problem definition, only need to find a solution now, which is the tricky part !