That is because of paths... When you visit /actions/inlichtingen_kalender.php it can find kalendergroot.php in the same /actions/ folder. But when you link it from your menu, the PHP is actually running directly from /index so it can't find /kalendergroot.php.
The easy way to fix it, for kalendergroot.php actually have the whole system path... ie: /home/your-username/kta-wollemarkt.be/public_html/actions/kalendergroot.php
PS: that's just an example of a full path, don't use that!
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