After days of trying and editing I finally found the cause. In the footer.php file I deleted this line: <?php wp_footer(); ?>
because I wanted to remove the floating footer (the “Proudly powered by WordPress Theme: Button 2” thing). That also somehow caused the menu to stop working. Now I need to find another way to remove the footer ??
@jarretc I’ve downloaded the theme and uploaded it to my self-hosted site. But thanks for the answer.
@fresatomica I already deactivated all of the plugins (which are jetpack and yoast SEO), and looked at the changes I made in the code. Couldn’t find anything that could have caused this, but maybe I should just start with a fresh installation of the theme again.