html in exerpts does not work

now i see – sorry – this is a portfolio grid – and you changed the grid-content to an overlay via custom css

you would have to hamper the Lightbox link of the inside link like this:

(function($){
  $('.grid-content a').on('click', function(e){
      e.preventDefault();
      var inlineLink = $(this).attr('href');
      window.open(inlineLink, '_blank');
  });
})(jQuery);

so it has the chance to open a normal link. we will see later on how to solve the link over background link problem

can you please try this in you quick css:
( all here with your custom-class – if you want that globaly – ask)

.referenzen .grid-content a {
  pointer-events: auto;
}

.referenzen .grid-content:hover {
  opacity: 1;
  transform: translate(-50%,-50%);
  transition: 1s all ease;
}

.referenzen .grid-image .image-overlay {
  z-index: 0 !important;
  display: none !important;
}

and this to your child-theme functions.php:
(and because this should only affect portfolio grids that are set to lightbox, here is the selector with grid-links-lightbox class.)

function open_non_lightbox_links_in_portfolio_grid(){
?>
<script type = "text/javascript">
window.addEventListener("DOMContentLoaded", function () { 
  (function($){
    $('.grid-links-lightbox .grid-entry  a:not([href^="#"])').on('click', function(e){
        e.preventDefault();
        var externalLink = $(this).attr('href');
        window.open(externalLink, '_blank');
    });
  })(jQuery);
});
</script>
<?php
}
add_action( 'wp_footer', 'open_non_lightbox_links_in_portfolio_grid' );

you see what the code does?
you have set the portfolio grid to open all links in a light box.
But your e-mail or links to external Links are not from the type: href=”#…” – the attribute selector above excludes these links to popups – and only prevents the function of the lightbox script where the links do not start with a hash (#).

PS: if you like to open those links not in an additonal window replace _blank with _self