Could not load feed.: This XML document is invalid

Trying to load a RSS from a Joomla site we get:

Could not load feed.: This XML document is invalid, likely due to invalid characters. XML error: Mismatched tag at line 6, column 16

But nothing strange at this line and w3c validator says “This is a valid RSS feed”

This is the beging of code of the RSS:

<?xml version='1.0' encoding='UTF-8'?>
<rss version="2.0" xmlns:atom="https://www.w3.org/2005/Atom">
<channel>
<title>XXXXXX</title>
<link>https://www.XXXXXXXXXX.com/</link>
<description><![CDATA[XXXXXXXX.]]></description>
<language>es-es</language>
<webMaster>[email protected] (XXXXXXXXXX.com)</webMaster>
<pubDate>Fri, 21 Feb 2020 20:33:32 +0100</pubDate>
<generator>JSitemap Pro</generator>
<atom:link rel="self" type="application/rss+xml" href="https://www.XXXXXXX.com/index.php?option=com_jmap&view=sitemap&format=rss&lang=es"/>
<item>
<title>el medio ambiente</title>
<link>https://www.XXXXXXXX.com/es/XXXXXXXXX-18372</link>
<guid isPermaLink="true">https://www.XXXXXXXXXX.com/es/XXXXXXXXX-18372</guid>
...