I've also encountered that one. It is irritating as I wonder why I don't see any new posts links and it takes me about a minute to figure it out.

I am an admin, and I'm not sure either, or at least I'm not sure it's a board problem.

The log in status when you click "remember me" is stored in your cookies, in ie there are settings that control this in the security and in the auto complete pages, so if your losing it, it's possible that is getting deleted. usually after deleting my web history I have to log in again myself, but that's just how things work.

There are certainly no admin settings pertaining to this system, it's part of punbb automatically, ---- as I said I use it myself and other than when updating history it tends to work fine.

Then again, there was a period when it refused to work at all, i think due to some changes microsoft had made to ie at the time, so I think it's just another case of microsoft's insane security, that our old mod Cx2 once described as putting five locks on your door then leaving the key outside under the mat .

I'm %95 sure that it's an ie problem with the storage of cookies, not a board problem.

As I said, there was an occasion when it decided to stop working for me too, even though i hadn't deleted anything, and even now I'm occasionally asked to log in. I'm not sure if ie automatically clears cookies or something like that, but it might be. You might try monkeying about in the auto complete and security pages of  your ie settings and seeing if that helps.