In further testing I believe I have narrowed this down. I can repeatedly recreate this message if I enter a valid INDI XREF (e.g. “I1234”) in the general search box at the top of each page (in the header).
Can you please test the attached replacement file to see if it also stops the recurrence of the error message on your system.
The file in the attached zip folder replaces the one of the same name (note the capital “S” for Search) in kiwitrees/library/WT/Controller/Search.php