no dropdown menu visible

Can you link to a page with the issue?

off course i can: https://www.helden-ag.de

to make the second level at least visible, i changed 1 line in the header.php to

<?php wp_list_pages(‘sort_column=post_date&depth=2&title_li=’); ?>
(orinally the depth was set to 1)

Okay, it doesn’t look like you have set up a Child Theme. If for what ever reason the theme updates your change will be erased. It’s also a lot easier to track and manage your changes in a Child Theme so it’s well worth setting one up: https://codex.www.ads-software.com/Child_Themes .

to make the second level at least visible, i changed 1 line in the header.php to

So, to make that change in your Child Theme all you need to do is copy the header.php file and paste it into your Child Theme folder. Then modify that file from within your Child Theme folder.

The CSS to make the dropdown menu behaviour is quite easy. You can add it, and any other CSS modification, to your Child Theme style.css file. Add this:

#nav ul ul {
 display: none;
}

#nav li:hover ul {
 display: block;
}

Fundamentally that’s all you need, however you may want this instead:

#nav li {
 position: relative;
}

#nav ul ul {
 display: none;
 position: absolute;
 left: 0;
 top: 3em;
}

#nav li:hover ul {
 display: block;
}

#nav li:hover ul {
 display: block;
}