It was simply stated “Impossible to decode” !
Unfortunately my experience with MailChimp was quite bad as I had to refresh several times the CAPTCHA I had to decode.
I like the post “Why You Should Stop Using CAPTCHAs” written by Oskar Smith stating the following: “we accept [the CAPTCHA] as an unavoidable step in the battle against bots and spam. But what if it was shown that CAPTCHA codes are not only damaging the usability of your website, but also hampering the ability of your site to create leads, generate sales or otherwise function and interact with your audience?“
I think that MailChimp goes definitely in the wrong direction….