Hello, I am not exactly sure how to replicate the error as Mailchimp mentioned it themselves.
Here is there email that they had sent:
"Thanks for reaching out to support. I definitely understand the frustration when an RSS feed doesn't validate and send as expected - I'm happy to help get you in the right direction here for how best to get things cleared up.
I went ahead and tested in an external validator, and it does indeed appear to be a valid feed. However, when checking into some things in our backend, it appears that our system is unable to connect with the feed - specifically, we're receiving a 403 forbidden message. This indicates that there may be some sort of issue where the feed is hosted preventing our system from accessing it. Some sort of change on the site's end is potentially what caused this, especially if it was working as expected previously.
What we would recommend in this case is to reach out to the web admin or host to see if they can offer any insight as to what may be causing that block. From the MailChimp side of things, we're a bit limited in the specific guidance we can offer to resolve those type of issues, but they should definitely be able to get you in the right direction for getting things worked out.
If you have any further questions or anything else we can help out with, don't hesitate to reach back out and we'll be glad to assist."
Not sure if any of this may help. I will disable the plugin and get in touch with Mailchimp again as well.