wp_update_post not working

I have the following code in a single script to be run one time to fix post names which are empty.

// WP_Query arguments
$args = array (
        'post_type'              => 'books',
        'name'                                   => '',
        'post_status'            => 'publish',
        'posts_per_page'                 => -1,
);
add_filter( 'posts_where', 'custom_posts_where' );
echo "1";
// The Query
$query = new WP_Query( $args );
echo "2";
$germanizer = new Germanizer();
// The Loop
if ( $query->have_posts() ) {
        while ( $query->have_posts() ) {
                $query->the_post();
                // do something
                $seo_slug   = seo_slugs( $germanizer->sanitize_title_filter( $post->post_title ) );
                // check the slug and run an update if necessary
                if ( $post->post_name != $seo_slug ) {
                        wp_update_post(
                        array (
                                'ID'        => $post->ID,
                                'post_name' => $seo_slug
                        )
                        );
                }
        }
}

function custom_posts_where( $where ) {
    remove_filter( current_filter(), __FUNCTION__ );
    if( FALSE === strpos( $where, 'post_name' ) ) {
        $where .= sprintf( " AND %s.post_name = '' ", $GLOBALS['wpdb']->posts );
    }
    return $where;
}

// Restore original Post Data
wp_reset_postdata();

I have included wp-config.php so that is not the problem. In fact the script executes without any error on my local development environment (MAMP) and prints 2 but does not update the post_name.

But on my test env, it echoes only 1 without any error and it naturally does not update the post_name. The total number of posts on my test env is around 5k, so I wonder if that is the reason.

This code uses Germanix Plugin by @toscho and might be relevant information.