Run composer require checker and solve issues.

src/Bundle/Resources/contao/templates/be_leaflet_about.html5

@@ -21,7 +21,7 @@
             </p>
         </div>
         <div class="right">
-            <a href="http://www.netzmacht.de" <?php echo LINK_NEW_WINDOW; ?>>
+            <a href="http://www.netzmacht.de" target="_blank">
                 <img src="bundles/netzmachtcontaoleaflet/img/logo.png"></a>
             <p>
                 <strong class="company">netzmacht </strong><br>