Mailchimp doesn’t work RSS not validated

I am trying to get MailChimp working, building a template which includes the last x (5 or 10) posts on my blog. I can’t get it to work, because the RSS isn’t validating.

See: https://validator.w3.org/feed/check.cgi?url=http%3A%2F%2Flekkerlezen.net%2Ffeed

I have already turned off every single plugin. To no avail.

Now what?

PS: not a real dev, so please speak real-people language (think trying to explain this to your grandmother)

edit:
line 39 is an article with a picture I uploaded.