I set Category X and Forum X inside it, to PRIVATE.
There is a usergroup xxx to match.
Usergroup xxx has general permissions to view / post in forums etc which they have no problem doing elsewhere.
On the Node Tree I selected the blue PERMISSIONS button for this forum.
As well as setting it to Private I selected usergroup xxx and selected GREEN permissions to view, post, edit their posts as normal.
This all worked OK some time ago.
The forum X has not been used for a while.
Recently I moved both the Category X and the Forum X away from root under another Category.
Members of usergroup xxx told me they can no longer access Forum X.
I logged in as one of them and I can see Category X but not Forum X inside it.
I moved both the Category X and the Forum X back to root on the node tree.
They still cannot access the forum!