Custom URL rewrite with add_rewrite_rule

Updating the rewrite-rules manually as follows worked for me:

 function gallery_rewriterules( $wp_rewrite ) {

  $new_rules = array( "hochzeitsfotografen/([^/]+)/([^/]+)/?([0-9] {1,})/?" => "index.php?fotograf=".$wp_rewrite->preg_index(1)."& gallery=".$wp_rewrite->preg_index(2)."&img=".$wp_rewrite->preg_index(3));
  $wp_rewrite->rules = $new_rules + $wp_rewrite->rules;  
  }
  add_filter('generate_rewrite_rules','gallery_rewriterules');