Come, Watson, come! Error with MailChimp

Hello!

I can no longer subscribe an email address into the subscription box nor the popup. I use MailChimp and the error I get is: Come, Watson, come! The game is afoot.
You’ve stumbled upon a missing page, but the evidence is elementary. We’re on the case.

I haven’t changed anything, it just started to happen. My website is https://harvardkennedyschoolreview.com/

I hope you can help me!

Many thanks,
Victoria