[Obersturmfuhrer]

I didn't get that error message but I got invisible metal clipping brushes in The Last Castle map. What I did was opened the map file in notepad and searched for the keyword "metalclip" and found that my map had a ton of these unseen brushes. So I copied down the brush numbers and went back into the editor, found the brushes, and deleted them. It was weird because when I plugged in the brush number, the 2D view immediately hovered over the "Z" point because there was no visible brush found. I can only assume either it was a hidden brush that would not unhide or that radiant still has some bugs in it that creates extra hidden brushes occasionally.

I hope this helps.