Hi Simon,
I believe, that on the General Options -> Sources 2 panel on the search instance options you have some values entered into the Exclude terms by ID field, here: https://i.imgur.com/4AIiYYr.png
The problem is, that you probably entered term/category names, but that input field expects IDs, thus causing the query to fail.
To get the a term ID, simply open it in the WordPress category editor screen, and on the browser URL bar, you should be able to see it, like so: https://i.imgur.com/BbAs0tg.png
Once you enter the category IDs, instead of the names, the error will go away.