What you need to do is release the loNode variable, the best place to do this is just before the ENDFOR, as putting it after will not work, I have come across this problem before with the ListView control.

So change you FOR-EACH to:

FOR EACH loNode IN ThisForm.oleTree.Nodes
    loNode.Selected = .F.
    loNode.Checked  = .F.
    m.NodeCounter   = loNode.Index
    loNode          = .NULL.
ENDFOR 

Not too sure what the exact issue is, but it seems like the loNode seems to keep a reference to the TreeView Node object, even though you cannot see this.