Only old 1.2 loop works in latest 1.5 version

Ik have the latest version of WP installed. When I use this old code:

if ($posts) : foreach ($posts as $post) : start_wp();

the page works, but when I use the new:

if ( have_posts() ) : while ( have_posts() ) : the_post();

I only get a blank page.

How can this be? I’m pointing to the correct wp_blog_header file.