"Show unread posts since last visit"

The lurker that I am, I use this link a lot. Though, just now, it stopped working. When I click it, it is blank. It finds no new topics, which is strange. I'm not that good at lurking (never read everything) and moreover, the forum itself thinks I haven't read everything because most subforums are red coloured (and not grey). Ima in trouble, somebody save me