So I don’t have any idea what I did or how this happened. Check out this site: https://vmedia.de/claire-3
Look carefully at the first slide in the carousel. The hamburger menu button is visible in the middle of the first slide. It is not visible in the second slide. If I resize the browser window to trigger the mobile menu, the hamburger icon is shown twice: once in the menu/navi, and once in the middle of the first slide.
I also can’t delete or move it in the layer navigator because it’s locked.
See screenshots …