What is JotForm?
JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
At JotForm, we want to make sure that you’re getting the online form builder help that you need. Our friendly customer support team is available 24/7.
We believe that if one user has a question, there could be more users who may have the same question. This is why many of our support forum threads are public and available to be searched and viewed. If you’d like help immediately, feel free to search for a similar question, or submit your question or concern.
I have tried to integrate my Mailchimp account to a Sample (Contact Us) Form and it worked without encountering any issues.
I have tried both in Google Chrome and in Mozilla Firefox.
I have also made a test submission and it also worked.
Please make sure that the API Key that is used to authenticate your Mailchimp account is correct. Also, please make sure that the List you're selecting still exist.