Yeah, here's the link again:I just tried the main Forums list page, and clicked Notebooks. It went here:
The difference between this URL and the one in post #4 is this one lacks a "forums/" element before "categories/".
Maybe an admin or @arn fixed this while we were yakking.
Can you revisit the main Forums list, do a Page Refresh/Reload in your browser, and then check what the URL of the "Notebooks" link/heading is?