icingaweb2/application/views/scripts/error/error.phtml

<?php if (! $hideControls) { ?>
  <div class="controls">
    <?= $this->tabs->showOnlyCloseButton() ?>
  </div>
<?php } ?>
<div class="content">
  <p><strong><?= nl2br($this->escape($message)) ?></strong></p>
<?php if (isset($stackTrace)) : ?>
  <hr />
  <pre><?= $this->escape($stackTrace) ?></pre>
<?php endif ?>
</div>