Main menu disappeared

Main menu is not visible on the page (near the top). When I reload the page, it is visible for a fraction of a second, then it disappears. I inspected the page HTML code, but it is beyond my skills.

The menu is defined and properly set – it has been there for more than a year. Also, it is visible in the admin in the “Customize a template” page.

Plugins I use: “All In One WP Security”, “Cookie banner plugin for WordPress”, “Elementor”, “GA Google Analytics”, “Polylang”, “Statistiky Burst” and “Yoast SEO”

The page I need help with: [log in to see the link]