Feedburner doesn’t work with WP-s feed

Hello!

I’ve now installed a new WP engine on my host. I want to use my old Google Feedburner service, but I can’t modify the old rss link in Feedburner, because it says: Error on line 578: The markup in the document following the root element must be well-formed.

The new RSS link is this: https://szekuriti1.fw.hu/?feed=rss2

And I have another problem, this message appears everywhere, and I can’t delete them: Warning: fsockopen() has been disabled for security reasons in /storages/storage1/s/sz/szekuriti1/public_html/wp-includes/class-snoopy.php on line 1142

What’s this?

Thanks!