When thew width of the window is narrowed, the main menu items are shown in 2 or more lines.
Then the submenu-items of the first line are covered by the items of the second line. So this submenu items are not selectable.

The 1. image shows the mainmenu in one line. So all submenu-items of "unser-Dorf" are selectable.
The 2. image show the narrowed main-menu (covers 2 lines now) and thje first submenu item of "usner-Dorf" is not selectable, because it is covered by the mainmenu item "Fotos" and "F-Schlange".
Solution: When submenu items are shown, they must have priority over the main menu items and must be selecable.

Hello,

Send a link to a page where we can see the problem.

Regards,
ARI Soft

You can see the problem on page: www.hasborn.de
When you narrow the window, so that the main menu must be displayed in more than one line, the some of sub menu items will be covered by the mainmenu items.

The sub menu items must always be on the highest layer.

Add the following CSS rule to "CSS Styles" parameter in module settings:

.ux-menu-sub {z-index:30000!important;}

Regards,
ARI Soft

Thanks for your answer. But I am new to joomla and I don't know, in which file I have to add the code line. I am using the protostar template.
Is it the ../template/protostar/css/template.css?

Open module settings page, find "CSS Styles" parameter and add the CSS rule to the parameter.

Regards,
ARI Soft