Update 2:
After setting ‘Override the default WordPress search results…’ to Off, I get an error message from the generic WP search when I use a search term that doesn’t occur on the site:
Warning: A non-numeric value encountered in /home/lvbhb/public_html/wp-content/themes/Avada/search.php on line 60 (and the same for lines 61 + 62)
Example: https://www.lvbhb.nl/?s=isserniet