I have to correct myself. After comparing source code of the page “without booking” and the page that is supposed to show an event with booking option, the event content is indeed missing. But anyway also the theme generated footer is missing, why I still think, there is a code issue somewhere, cutting everything below, while generating the page.
still very confused. the other pages, generated automatically by the plugin are working (until the point, wehere the event page is displayed)
see summary page for example: https://www.smarter-franke.de/veranstaltungen/