Update: After some digging I realized, that the action hook 'publish_post' does not fire, when a custom post type is published. One has to use another hook like 'publish_{{custom_post_type}}' to catch the publishing and kick-on the push to the hub.
In my case I added
add_action( 'publish_news', array( 'PubSubHubbub_Plugin', 'publish_post' ) );
to the init() funtion in pubsubhubbub.php
