This does not work for me. Just to test, about an hour ago I marked all forums read. I came back and now it says 83 and when I click it, it shows this. Or maybe it means mark all forums is not working? LOL But something is not right, I am guessing its some type of cookie problem? I do not know.
Apologies if I've misinterpreted things but there could be 83 new posts and only one thread listed if all 83 new posts were made in that one thread. It's a function I use habitually and I've not noticed an issue with it here but I'll keep a close eye on it.