URL default encoding should match rules for href's.   VS &

application/layouts/scripts/parts/navigation.phtml

@@ -13,7 +13,7 @@ $url = Url::fromRequest()->remove('_render')->getRelativeUrl();
 $menu = Menu::fromConfig();
 ?>
 <div id="menu" data-base-target="_main">
-  <form action="<?= $this->href('search') ?>" method="post">
+  <form action="<?= $this->href('search') ?>" method="get">
     <input type="text" name="q" class="search" placeholder="Search..." autocomplete="off" autocorrect="off" autocapitalize="off" spellcheck="false" />
   </form>
 <?= $this->partial('parts/menu.phtml', array(

library/Icinga/Web/Url.php

@@ -256,7 +256,7 @@ class Url
         if (empty($this->params)) {
             return ltrim($this->path, '/') . $this->anchor;
         }
-        return ltrim($this->path, '/') . '?' . http_build_query($this->params) . $this->anchor;
+        return ltrim($this->path, '/') . '?' . http_build_query($this->params, '', '&') . $this->anchor;
     }
 
     /**