The custom field configuration was incorrect.
On the Frontend search Settings -> Custom fields you have a Uitleenbaar custom field selector created.
This was using the “_uitleenbaar” custom field, which is not defined on any post, so it didn't return any results. I have selected the “uitlenbaar” insteand and I have started getting results.
Some post may not show yet, as the “uitlenbaar” custom field is not yet defined on them. You will have to edit those posts and save them to have the “uitlenbaar” custom field set.Best,
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