Can you try adding this to your/a child theme functions.php file?
if ( ! function_exists( 'sf_post_detail_heading' ) ) {
function sf_post_detail_heading() {
global $post;
?>
<header class="article-heading hidden-hatom">
<div class="container">
<div class="entry-title" itemprop="name"><?php the_title(); ?></div>
<span class="date updated"><?php the_date(); ?></span>
<span class="vcard author">
<span class="fn"><?php the_author_meta( 'display_name' ); ?></span>
</span>
</div>
</header>
<?php
}
}
add_action( 'sf_post_article_start', 'sf_post_detail_heading', 10 );
Unable to replicate the error locally, so difficult to test. Hopefully that sorts it.
– Ed