Hustle problem with Mailchimp

My Hustle pop-up shows after three seconds on the home page, but, alas, when I try to fill in first and last names and email address and hit Sign Up, I get a Something went wrong, please try again message.

In the backend of Wordpress, I see that there may be a problem with Hustle connecting to Mailchimp, as it won't generate any lists and gives the same error as above. I tried deactivating all plugins except Hustle and that did nothing.

I can send you the actual list name and its unique identifier if you'd like. BTW, I checked to make sure the API key was correct and it is, in fact, I generated another API key and tried that as well, to no avail.

I've activated Support. Can you help?

Best,

Eric