RSS Error in feed with atom / unbound prefix - rss

Question

I'm trying to fix my RSS feed and I can't figure out why it says unbound prefix. I tried removing it and searching for different answers. I saw that most things should be output using CDATA. But that does not seem to be the issue here. I can also see that some title's have their special characters such as ' converted, but nothing that should result in a feed failure.
Here's my code:
<?php
header( 'Content-Type: ' . feed_content_type( 'rss2' ) . '; charset=' . get_option( 'blog_charset' ), true );
$more = 1;
echo '<?xml version="1.0" encoding="' . get_option( 'blog_charset' ) . '"?' . '>';
do_action( 'rss_tag_pre', 'rss2' );
?>
<rss version="2.0" xmlns:hst="http://hirstart.hu/rss-mod" <?php do_action( 'rss2_ns' ); ?>>
<channel>
<title><?php // wp_title_rss(); ?> Tőzsdefórum | Minden ami tőzsde</title>
<atom:link href="<?php self_link(); ?>" rel="self" type="application/rss+xml" />
<link><?php bloginfo_rss( 'url' ); ?></link>
<description><?php bloginfo_rss( 'description' ); ?></description>
<lastBuildDate><?php echo get_feed_build_date( 'r' ); ?></lastBuildDate>
<language><?php bloginfo_rss( 'language' ); ?></language>
<sy:updatePeriod>
<?php
$duration = 'hourly';
echo apply_filters( 'rss_update_period', $duration );
?>
</sy:updatePeriod>
<sy:updateFrequency>
<?php
$frequency = '1';
echo apply_filters( 'rss_update_frequency', $frequency );
?>
</sy:updateFrequency>
<?php
do_action( 'rss2_head' );
while ( have_posts() ) :
the_post();
?>
<item>
<title><?php the_title_rss(); ?></title>
<?php
$post_id = get_the_ID();
$kiemelt = get_field( 'kiemelt_cikk' );
if ( is_array( $kiemelt ) ) {
if ( in_array( 'igen', $kiemelt ) ) {
$output = '<hst:titlepage>1</hst:titlepage>';
echo $output;
}
}
?>
<link><?php the_permalink_rss(); ?></link>
<?php if ( get_comments_number() || comments_open() ) : ?>
<comments><?php comments_link_feed(); ?></comments>
<?php endif; ?>
<dc:creator>
<![CDATA[<?php the_author(); ?>]]>
</dc:creator>
<image>
<?php
if ( has_post_thumbnail( $post->ID ) ) {
the_post_thumbnail_url();
}
?>
</image>
<pubDate><?php echo mysql2date( 'D, d M Y H:i:s +0000', get_post_time( 'Y-m-d H:i:s', true ), false ); ?></pubDate>
<?php the_category_rss( 'rss2' ); ?>
<guid isPermaLink="false"><?php the_guid(); ?></guid>
<?php if ( get_option( 'rss_use_excerpt' ) ) : ?>
<description>
<![CDATA[<?php the_excerpt_rss(); ?>]]>
</description>
<?php else : ?>
<description>
<![CDATA[<?php the_excerpt_rss(); ?>]]>
</description>
<?php $content = get_the_content_feed( 'rss2' ); ?>
<?php if ( strlen( $content ) > 0 ) : ?>
<content:encoded>
<![CDATA[<?php echo $content; ?>]]>
</content:encoded>
<?php else : ?>
<content:encoded>
<![CDATA[<?php the_excerpt_rss(); ?>]]>
</content:encoded>
<?php endif; ?>
<?php endif; ?>
<?php if ( get_comments_number() || comments_open() ) : ?>
<wfw:commentRss><?php echo esc_url( get_post_comments_feed_link( null, 'rss2' ) ); ?></wfw:commentRss>
<slash:comments><?php echo get_comments_number(); ?></slash:comments>
<?php endif; ?>
<?php rss_enclosure(); ?>
<?php
do_action( 'rss2_item' );
?>
</item>
<?php endwhile; ?>
</channel>
When I try to validate, I get this: https://validator.w3.org/feed/check.cgi?url=https%3A%2F%2Ftozsdeforum.hu%2Ffeed%2F
Actual feed link is: https://tozsdeforum.hu/feed/
Any suggestions are appreciated.