I found the specific problem with the links in the module:
In the subpage with only one category the link looks like this
.. /index.php/bardolino-startpage-de/642-beitraege-bardolino/542-bardolino
at the startpage with combined categories the link looks like this
.. /index.php/642-beitraege-bardolino/542-bardolino
The part "bardolino-startpage-de/" is missing and Joomla displays my startpage.