Strange RSS error

I’m running into a really weird error that has me at a loss. I’m working on a wordpress site and I’m trying to read from an RSS feed but I keep getting

Error RSS: This XML document is invalid, likely due to invalid characters. XML error: Mismatched tag at line 12, column 8

The problem is that I have checked the RSS feed on https://validator.w3.org/feed/ and it is a valid feed. The same thing has happened with other feed sources, and the error is always at line 12, column 8. Not only that, if I keep hitting the refresh button on the browser the line 12 changes to line 109 sometimes and then back again to 12. But there’s more, one of the feeds that I’m testing doesn’t even have more than 100 lines and it stills shows the error on line 109 sometimes (when I keep constantly refreshing the page). Also the original feed which I ultimately want to use on line 12 only has this: <item>, which means there’s no column 8. I’ve tested this on 3 separate installations of WordPress, using different themes (even the built in twenty seventeen) and different plugins, and versions of WordPress, 4.7 and 4.7.4.

I can’t provide a url for the site because it is hosted on an intranet, and the other tests I’ve done have been with local installs. But here is the xml for the rss, if you test it on a validator you’ll see it’s valid.

<?xml version='1.0' encoding='UTF-8' ?>
<rss version='2.0' xmlns:atom='http://www.w3.org/2005/Atom'>
    <channel>
        <title>Online Resource Portal</title>
        <link>http://www.intranetportal.local/index.php?P=Home</link>
        <description>Channel Description</description>
        <atom:link href='http://www.intranetportal.local/index.php?P=RSS' rel='self' type='application/rss+xml' />
        <language>es-es</language>
        <pubDate>Thu, 19 Oct 2017 12:00:00 -0400</pubDate>
        <lastBuildDate>Mon, 10 Apr 2017 12:00:00 -0400</lastBuildDate>
        <docs>http://www.rssboard.org/rss-2-0-1</docs>
        <item>
            <title>Laboratorio Clínico</title>
            <link>http://www.intranetportal.local/index.php?P=FullRecord&ResourceId=6</link>
            <guid>http://www.intranetportal.local/index.php?P=FullRecord&ResourceId=6</guid>
            <description>Laboratorio Clínico</description>
            <pubDate>Wed, 31 Dec 1969 07:00:00 -0500</pubDate>
        </item>
        <item>
            <title>Familia y Juventud</title>
            <link>http://www.intranetportal.local/index.php?P=FullRecord&ResourceId=5</link>
            <guid>http://www.intranetportal.local/index.php?P=FullRecord&ResourceId=5</guid>
            <description>Familia y Juventud</description>
            <pubDate>Wed, 31 Dec 1969 07:00:00 -0500</pubDate>
        </item>
        <item>
            <title>Diabetes Mellitus en el Embarazo</title>
            <link>http://www.intranetportal.local/index.php?P=FullRecord&ResourceId=4</link>
            <guid>http://www.intranetportal.local/index.php?P=FullRecord&ResourceId=4</guid>
            <description>Diabetes Mellitus en el Embarazo</description>
            <pubDate>Wed, 31 Dec 1969 07:00:00 -0500</pubDate>
        </item>
        <item>
            <title>Baja Visión</title>
            <link>http://www.intranetportal.local/index.php?P=FullRecord&ResourceId=3</link>
            <guid>http://www.intranetportal.local/index.php?P=FullRecord&ResourceId=3</guid>
            <description>Baja Visión</description>
            <pubDate>Wed, 31 Dec 1969 07:00:00 -0500</pubDate>
        </item>
        <item>
            <title>Glaucoma</title>
            <link>http://www.intranetportal.local/index.php?P=FullRecord&ResourceId=2</link>
            <guid>http://www.intranetportal.local/index.php?P=FullRecord&ResourceId=2</guid>
            <description>Glaucoma</description>
            <pubDate>Wed, 31 Dec 1969 07:00:00 -0500</pubDate>
        </item>
        <item>
            <title>Adulto Mayor</title>
            <link>http://www.intranetportal.local/index.php?P=FullRecord&ResourceId=1</link>
            <guid>http://www.intranetportal.local/index.php?P=FullRecord&ResourceId=1</guid>
            <description>Adulto Mayor</description>
            <pubDate>Wed, 31 Dec 1969 07:00:00 -0500</pubDate>
        </item>
    </channel>
</rss>

Anybody has a clue on what might be going on.