I have improved your code to a new version as i have found that you have used text “AGB” out of anchor tag
Updated Code
$html="";
$html .= '<div class="zerif-copyright-box">';
$html .= '<a class="zerif-copyright" href="http://themeisle.com/themes/zerif-lite/"'.$attribut_new_tab.' rel="nofollow">'.__('AGB','zerif-lite').'</a>';
$html .= '<a class="zerif-copyright" href="" "'.$attribut_new_tab.' rel="nofollow"> Impressum</a>';
$html .= '</div>';
$html .= '</div>';
echo $html;
Hope this helps!