Because of a previous error the article St. Georg (Nördlingen) showed an empty page. An action=purge fixed this. The HoverCard is still not shown because the TextExtract is empty: "extract":"" (https://de.wikipedia.org/w/api.php?action=query&format=json&prop=extracts|pageimages|revisions&formatversion=2&redirects=true&exintro=true&exsentences=5&explaintext=true&piprop=thumbnail&pithumbsize=300&rvprop=timestamp&titles=St._Georg_%28N%C3%B6rdlingen%29&smaxage=300&maxage=300&uselang=content) TextExtract has it own cache.
To repair such problems it would be useful to clear the TextExtract cache on action=purge.
The function cacheKey() in ApiQueryExtracts.php has to include something from WikiPage which get changed on doPurge().