The problem occurs when you click on a link to a response to a post (i.e. any post that isn't the original in a thread). We're working on it

(Edit: The problem is being fixed right now. For the time being, the 404 errors don't exist anymore, but when you click on a thread's hyperlink, the page loads and then immediately redirects to the main index. The problem will be solved, but for the time being, all that you have to do is click the 'back' button in your browser after the main index loads and you'll be at the page that you want.)