I've never understood what forum users or admins had against old threads. Isn't the idea of a forum to facilitate conversations? Why should it matter if a user adds a reply to a thread that's, say, a year old?
I use APC but I reset it and the warning is still there... EDIT: My fault! I double checked in the Admin Control Panel and the addon was still activate. I was sure I deactivated it, maybe I closed the tab before it was done.
The other problem, as @Xon said, is still there, it doesn't calculate from "last post date".