Error message on my blogs RSS Feed

Try deactivating/deleting the AddThis Button plugin. It’s inserting page markup in your RSS feed.

I tried that and the message is still there.

Are you sure that you have completely removed the AddThisButton plugin? Only the code is still there in your RSS feed:

<!-- AddThis Button BEGIN -->
<div><div class="addthis_toolbox addthis_default_style">
<a href="https://www.addthis.com/bookmark.php?v=250&username=juliannah" class="addthis_button_compact">Share</a>
<span class="addthis_separator">|</span>
<a class="addthis_button_facebook"></a>

<a class="addthis_button_myspace"></a>
<a class="addthis_button_google"></a>
<a class="addthis_button_twitter"></a>
</div>
<script type="text/javascript" src="https://s7.addthis.com/js/250/addthis_widget.js#username=juliannah"></script></div>
<!-- AddThis Button END -->

Hi,
I am getting the same message :

XML Parsing Error: XML or text declaration not at start of entity
Location: https://localhost.test/wpmu/feed
Line Number 2, Column 1:<?xml version="1.0" encoding="UTF-8"?>
^

Feed https://ecoflare.eu/feed/

I’ve tried to remove the Addthis button from my templates (it was hardcoded) but it did not help.
Any ideas, what is wrong with the rss?

It was working perfectly in the v. 2.8.6, it just stopped to work after the automatic upgrade to 2.9.1

Thank you for any suggestions.

ok, I got it, it was one of my own plugins, have to debug it.