Slide In plugin messing with RSS feed

Have a problem with my RSS Feed since installing Slide In plugin. As soon as I turn off the plugin it works fine.

feed is: https://www.theglobalcmo.com/feed/?cat=10

getting error:
"error on line 16 at column 12: XML declaration allowed only at the start of the document

Below is a rendering of the page up to the first error."
(then the start of the Slide In content - starting with 'close', which i guess is the close button for it).

<div id="wdsi-slide_in" style="display:none;" class="wdsi-slide  wdsi-slide-left wdsi-slide-minimal wdsi-slide-minimal-light wdsi-slide-minimal-red" data-slidein-start="1%"  data-slidein-end="100%" data-slidein-after="0s" data-slidein-timeout="30s" data-slidein-id="1605" >

	<div class="wdsi-slide-wrap" style="width:600px;" >
		<div class="wdsi-slide-control">
	<div class="wdsi-slide-share wdsi-clearfix">
			</div>
	<div class="wdsi-slide-close"><a href="#">Close</a></div>
</div>		<div class="wdsi-slide-content">
			<h1 class="wdsi-slide-title wdsi-slide-bold wdsi-slide-italic">Secure Your Free Subcription Today</h1>
			<p><img class=" wp-image-1599 alignleft" style="padding-right: 10px; padding-bottom: 10px;" alt="Global CMO May 2013" src="https://www.theglobalcmo.com/wp-content/uploads/2013/05/GlobalCMO_May2013-1.jpg" width="175" height="248" /></p>
<p style="text-align: left;">Global CMO The Magazine brings to you the very latest and best practices in Marketing. Bridging the gap between Marketing theory and Marketing practice, featuring thought-leadership, the latest research, upcoming events and programmes and case studies, each designed to inspire, educate and inform.</p>
<p style="text-align: left;">Make sure you get your Monthly fix of Marketing insights, with the official magazine of Global Marketing Network.</p>
<p style="text-align: center;"><iframe src="http://widget.mailjet.com/7f57ae6d0ff88573a30097b246a0cb027bf532eb.html" height="138" width="310" frameborder="0" marginwidth="0" marginheight="0" scrolling="no"></iframe></p>
		</div>
			</div>
</div><?xml version="1.0" encoding="UTF-8"?>
<rss version="2.0"

All that stuff above the XML version shouldn't be showing up on the feed I think.

RSS actually works fine in Firefox on Mac (the Slide In code doesn't show up the top in the Source code), but it errors out in Safari, Chrome and IE,