I'm trying for hours now to find the error moving back and forth. I have a custom post type. Everything works fine until I add my custom meta box. Now suddenly the slug gets the title of one of $post->ID == '21' child pages instead its own title when creating a new custom post. It always is the title from the page with the lowest $post->ID. But when I remove my custom meta box again. Everything works fine. I may have tracked it down to the foreach but I can't see what's wrong there.

Besides this %postname% error everything's fine. The custom field gets saved correctly. I even can edit the slug manually. But that's not how it is supposed to be. Please help!

add_action( 'init', 'cpt_init_cases' );
function cpt_init_cases() {
  register_post_type( 'case', array(
    'labels' => array(
      'name' => __( 'Cases' ),
      'singular_name' => __( 'Case' )),
    'description' => 'Create cases to be displayed on single customer child pages',
    'public' => true,
    'menu_position' => 6,
    'rewrite' => array('slug' => 'case'),
    'supports' => array('title', 'editor', 'thumbnail', 'custom-fields')
  ));
}

/*** when I delete everything below this line slugging works as expected ***/
add_action('admin_init', 'admin_init');
function admin_init(){
    add_meta_box('refInfo-meta', 'Reference', 'reference_meta_callback', 'case', 'side', 'high');
}  

function reference_meta_callback( $post ){

  global $post;

  $custom = get_post_custom($post->ID);
  if( isset($custom['reference_id'][0]) ) {
    $reference_id = $custom['reference_id'][0];
  } else {
    $reference_id = '0';
  }

  wp_nonce_field( 'my_meta_box_nonce', 'meta_box_nonce' );

  ?>
    <form action="<?php bloginfo('url'); ?>" method="get">
      <select name="ref_id" id="ref_id">
        <option value="0" <?php selected($reference_id, '0'); ?>>- choose client -</option>

      <?php
      $args = array( 'numberposts' => -1, 'post_type' => 'page', 'post_parent' => 21);
      $posts = get_posts( $args );
      foreach( $posts as $post ) : setup_postdata( $post ); ?>
        <option value="<?php echo $post->ID; ?>" <?php selected($reference_id, $post->ID); ?>><?php the_title(); ?></option>
      <?php endforeach; ?>

      </select>
    </form>
  <?php
}  

add_action( 'save_post', 'save_reference_id' );  
function save_reference_id( $post_id ) {

  global $post;

  if( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) return;

  if( !isset( $_POST['meta_box_nonce'] ) || !wp_verify_nonce( $_POST['meta_box_nonce'], 'my_meta_box_nonce' ) ) return;

  if( isset($_POST['ref_id']) ) {
    update_post_meta($post->ID, 'reference_id', $_POST['ref_id']);
  }
}

Tried it without the <form> tags now. Problem still persists.