“Previous posts” results in a 404 error

My blog is set to show a week in every page (structure /page/x), with a link to previuous posts below the entry area. But on the last page, when just a post is shown, there is a link to non-existant previous posts that results in a 404 error. I read that someone else had the same problem, but, after having tried every proposed solution, the problem never got fixed. I use WP 1.5.1.1.