I have the elvira template here and can't open the modal box for the privacy statement.

I did it on another website with the Scarlett template. In the templates the override default_form.php had to be changed from $item->input to $item->renderfield(). Then I have adopted the CSS div.modal into the custom-css.

div.modal {
    position: fixed;
    top: 5%;
    left: 50%;
    z-index: 1050;
    width: 80%;
    margin-left: -40%;
    background-color: #fff;
    border: 1px solid #999;
    border: 1px solid rgba(0,0,0,0.3);
    *border: 1px solid #999;
    -webkit-border-radius: 6px;
    -moz-border-radius: 6px;
    border-radius: 6px;
    -webkit-box-shadow: 0 3px 7px rgba(0,0,0,0.3);
    -moz-box-shadow: 0 3px 7px rgba(0,0,0,0.3);
    box-shadow: 0 3px 7px rgba(0,0,0,0.3);
    -webkit-background-clip: padding-box;
    -moz-background-clip: padding-box;
    background-clip: padding-box;
    outline: none;
}

With the elvira template I don't find the error in the CSS. The "z-index" is here not work.

Link to the website with elvira-template: https://it-conserv.de/kontakt.html
link to the website with scarlett-template: https://demo.moorschreber.de/index.php/kontakt

Can you find the problem?

The field is the new field in the joomla-core with version 3.9