submenu header menu disappears when also using topmenu

I combine a topmenu and a header menu with submenu on my site.
On mouse-over, the submenu disappears most of the time (but not always, though there is no way of telling when it does or does not).

On mouse-over, right under the menu-item in the header menu, there is a thin line visible from the background of the main content (like the submenu is floating, instead of being attached to the header menu).

The problem does not occur when I disable the topmenu: in that case everything works fine.
I also tried (without the desired effect)
* disabling all plugins
* changing from child theme to original Hueman theme

I wonder if there is some margin setting for the topmenu that affects the header menu that I can adapt in the theme editor, or if I should use css class in the items for the top menu (and with what function). Or is there another solution for this?