can you see what you have in the sfmeta table.. for the forum ranks… I suspect its fine, but want to check…
ran some tests this afternoon and it worked as expected for me… and just to check, on the same admin panel, what is the single membership option set to? and do you have any special forum ranks?
Late reply, had to update my membership – never got the time, but here I am again 
Let’s start from the beginning again.
This is my forum-ranks:
It’s been working quite well… As soon as the members hits 90 posts, they get access to the yellow marked usergroup. But now, suddenly new members with only one post gets access to this group..
I don’t understand what you want me to give you from the sfmeta-table?
suddenly? so what’s changed?
how do you have new users mapped?
any membership plugins?