WordPress – 404 error on custom post type single pages

I use a custom post type, which also should have a custom taxonomy. When I visit the archive page /kollektion/ I can see all items of this CPT, when I visit /kollektion/mytaxonomy1/ I can see all items which are assigned to this taxonomy.

When I click on the single item of the custom post type, I get a 404 error. I also tried to go into my WP dashboard and change and resave the permalinks again – with no success.

Does anybody see an error in my declaration which causes the 404 error on single pages of my custom post type?

Thanks.

functions.php: Taxonomy

// register two taxonomies to go with the post type
function sm_register_taxonomy() {
    // set up labels
    $labels = array(
        'name'              => 'Kollektionen Categories',
        'singular_name'     => 'Kollektion Category',
        'search_items'      => 'Search Kollektion Categories',
        'all_items'         => 'All Kollektion Categories',
        'edit_item'         => 'Edit Kollektion Category',
        'update_item'       => 'Update Kollektion Category',
        'add_new_item'      => 'Add New Kollektion Category',
        'new_item_name'     => 'New Kollektion Category',
        'menu_name'         => 'Kollektion Categories'
    );
    // register taxonomy
    register_taxonomy( 'kollektionen', 'kollektion', array(
        'hierarchical' => true,
        'labels' => $labels,
        'query_var' => true,
        'show_admin_column' => true,
        'rewrite' => array(
            'slug' => 'kollektion', // This controls the base slug that will display before each term
            'with_front' => false // Don't display the category base before
        ),
    ) );
}
add_action( 'init', 'sm_register_taxonomy' );

functions.php: Custom post type

// Register Custom Post Type Kollektion
    function post_type_kollektion() {

        $labels = array(
            'name'                  => _x( 'Kollektionen', 'Post Type General Name', 'genesis' ),
            'singular_name'         => _x( 'Kollektion', 'Post Type Singular Name', 'genesis' ),
            'menu_name'             => __( 'Kollektion', 'genesis' ),
            'name_admin_bar'        => __( 'Kollektion', 'genesis' ),
            'parent_item_colon'     => __( 'Parent Item:', 'genesis' ),
            'all_items'             => __( 'All Items', 'genesis' ),
            'add_new_item'          => __( 'Add New Item', 'genesis' ),
            'add_new'               => __( 'Add New', 'genesis' ),
            'new_item'              => __( 'New Item', 'genesis' ),
            'edit_item'             => __( 'Edit Item', 'genesis' ),
            'update_item'           => __( 'Update Item', 'genesis' ),
            'view_item'             => __( 'View Item', 'genesis' ),
            'search_items'          => __( 'Search Item', 'genesis' ),
            'not_found'             => __( 'Not found', 'genesis' ),
            'not_found_in_trash'    => __( 'Not found in Trash', 'genesis' ),
            'items_list'            => __( 'Items list', 'genesis' ),
            'items_list_navigation' => __( 'Items list navigation', 'genesis' ),
            'filter_items_list'     => __( 'Filter items list', 'genesis' ),
        );
        $args = array(
            'label'                 => __( 'Kollektion', 'genesis' ),
            'description'           => __( 'Kollektionen', 'genesis' ),
            'labels'                => $labels,
            'supports'              => array( 'title', 'editor', 'excerpt', 'author', 'thumbnail', ),
            'taxonomies'            => array( 'post_tag', 'kollektionen' ),
            'hierarchical'          => false,
            'public'                => true,
            'show_ui'               => true,
            'show_in_menu'          => true,
            'menu_position'         => 5,
            'show_in_admin_bar'     => true,
            'show_in_nav_menus'     => true,
            'can_export'            => true,
            'has_archive'           => 'kollektion',
            'exclude_from_search'   => false,
            'publicly_queryable'    => true,
            'capability_type'       => 'page',
        );
        register_post_type( 'kollektion', $args );

    }
    add_action( 'init', 'post_type_kollektion');
}

function namespace_add_custom_types( $query ) {
  if( is_category() || is_tag() && empty( $query->query_vars['suppress_filters'] ) ) {
    $query->set( 'post_type', array(
     'post', 'nav_menu_item', 'kollektion', 'kollektionen'
        ));
      return $query;
    }
}
add_filter( 'pre_get_posts', 'namespace_add_custom_types' );