My wp-rss2.php and wp-atom.php validated, but wp-rss.php did not. I received the XML error described above. My (simplistic?) solution was to change line 31 of wp-rss.php (based partly on https://mosquito.www.ads-software.com/view.php?id=1132 ) from
<description><?php the_content_rss('', 0, '', get_settings('rss_excerpt_length')) ?></description>
to
<description><![CDATA[<?php the_content_rss('', 0, '', get_settings('rss_excerpt_length')) ?>]]></description>
Maybe I’m naive, but it just looks like this was missed in the orig. Hope this helps and if I’m totally wrong, let me know.