Hi, in blog_item.html.twig there’s a small omission in the lower nextSibling / prevSibling navigation.
Please change
<span class="post-title" {% if page.nextSibling.media.images %}style="color: #fff;"{% endif %}>{{ page.header.title }}</span>
to
<span class="post-title" {% if page.nextSibling.media.images %}style="color: #fff;"{% endif %}>{{ page.nextSibling.header.title }}</span>
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