white page after php code insert for custom post type [closed]

Question

You forgot about endwhile; at the end of loop.

Using template don’t forget to get_header() and get_footer()

Sample:

<?php
/*
  Template Name: Test
 */
?>

<?php get_header(); ?>

<div class="infoboxen">
    <div class="box">
        <?php $loop = new WP_Query(array('post_type' => 'drei_boxen', 'posts_per_page' => 10)); ?>

        <?php while ($loop->have_posts()) : $loop->the_post();
        endwhile; ?>                     
    </div>
</div>

<?php get_footer(); ?>