Menubar does not show submenus and no arrow

I am working with Drupal Version 10.2.6 and PHP Version 8.1.28

Menu item with submenu is activated.
checkbox to show subitems is clicked.
Submenu items are child items.

But there is no arrow shown and no submenu on mouseover or click.
only the highlighting/color change is working with mouseover.

no error message in protocoll.

I tired it with different browsers.