Problem with forms – error: Sorry, we couldn’t connect to the MailPoet server

Hi,
I have a premium version of MailPoet 3.
I encoutered a problem with the subscription form I have on my website (https://www.mediaroad.eu/category/mr-blog/).
When a user tries to subscribe to several newsletter lists, an error message appears: “Sorry, but we couldn’t connect to the MailPoet server. Please refresh the web page and try again.”
It happens in all browsers.
The only instance when this problem doesn’t occur is when someone is logged in to the website – eg. if I’m logged in as an admin to WP dashboard and I’m testing this form, I don’t get the error message.
Could you please help me with this?
Thanks.

The page I need help with: [log in to see the link]