I have found the problem: Check your diagnostics, and you will find that PHP
mbstring extension is missing.
ruuddesoet.nl/?diagnostics
This extension is responsible for encoding of "multibyte characters", for example ë in "België". That explains why some of your pages are choking when you have included these characters, for example the "Doel" page ruuddesoet.nl/content/2.Recent/4.Doel/page.json.
Really you should expect better from your hosting since mbstring is a core extension of PHP. If they turned it off at some point, or forget to install it when they upgraded PHP, I don't know.