It seems to be Mailchimp issue. Here’s the reply I got from them:
Thanks for writing into MailChimp Support! I’ll be happy to assist you as much as possible with your RSS feed.
You concern is certainly something that we’re aware of here at MailChimp, and our developers are aggressively working towards a resolution to this issue. While we are still working on a fix, you can definitely use a RSS Feed Mixer with MailChimp so your feed will continue work. What the Feed Mixer will do is act as a redirect for your feed url and will generate a new usable url link that you can use within MailChimp. Here is some resources on finding the right Feed Mixer for you: https://www.google.com/#q=rss+feed+mixer+