If you visit https://ilp-kit.michielbdejong.com/ and then open https://ilp-kit.michielbdejong.com/api/config in your browser, you will see a 'Not found' page. But if you load https://ilp-kit.michielbdejong.com/ in a private browsing tab, you see a JSON doc as expected. curl also obviously shows the JSON doc as expected.
I just realized this must be due to the service worker. We should mention this in the dev docs.