The breadcrumbs showed the page Element twice in the breadcrumbs. Finally I've found that in Drupal 8 the page title as the final element is handled by Drupal. The Theme D8W3CSS however has a proprocess_breadcrumb-function which adds the page-title if it's not empty. Thus the title shows up twice.
Solution: deactivating the proprocess_breadcrumb-function in the theme D8W3CSS