Thank you!
I tried deactivating the plugin, to see if there is anything on the search results page – but it’s empty as well for any search phrase: https://i.imgur.com/V6cmB6E.png
The issue seems to be, that the search results loop is missing from the theme search.php file. If you changed that file, then I suggest making sure to revert it back to the original where the posts loop is present. Otherwise WordPress will not show anything on the page.