icingaweb2/application/layouts/scripts/guest-error.phtml

<div id="guest-error">
    <div class="centered-ghost">
        <div class="centered-content">
            <div id="icinga-logo" aria-hidden="true"></div>
            <div id="guest-error-message">
                <?= $this->render('inline.phtml') ?>
            </div>
        </div>
    </div>
</div>