RSS Feed not working

Since two weeks, our MailChimp RSS is not working properly because of the Series Plugin. The Campaign does not send because of the issue. I deactivated the plugin and it works now.

This is the error from RSS Feed Validator:
This feed is valid, but interoperability with the widest range of feed readers could be improved by implementing the following recommendations.
line 1, column 38: Use of unknown namespace: https://publishpress.com/
<?xml version=”1.0″ encoding=”UTF-8″?><rss version=”2.0″
^