Problem solved, you were completely right. The other plugin led to the conflict. Your Plugin WP Show posts works excellent. Many thanks!
Small follow-up question: is ist possible to change the color of the “more text” (mehr lesen) box, e.g. get rid of the black border, fill it green and make the text white? And make the headline a bit smaller, e.g. H2 oder H3?
Otherwise just brilliant work and outstanding customer support !!