Hi @COMO-BRAM,
Here's the culprit on Line 202 of your code:
<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
Replace jQuery script above with one from a secure HTTPS server and it should restore the hamburger menu to working order.
<script
src="https://code.jquery.com/jquery-3.6.0.min.js"
integrity="sha256-/xUj+3OJU5yExlq6GSYGSHk7tPXikynS7ogEvDej/m4="
crossorigin="anonymous"></script>
jQuery 3.6 is the current version and may not be compatible with your outdated menu plugin. It's also worth noting that the code thoughout your site is outdated by today's standards -- e.g. HTML5 code replaced XHTML transitional. Legacy Fluid Grid layouts were replaced with Bootstrap framework and so on...
Read chapters, do code exercises and take quizzes at the end.
- https://www.w3schools.com/html/
- https://www.w3schools.com/css/
- https://www.w3schools.com/js/
- https://www.w3schools.com/bootstrap4/
Hope that helps.