There are many times when the problem is caused by caching services running either on the server or from 3rd party caching plugins.
Clearing the cache from those plugins or re-fetching the MailChimp lists will fix the problem in this case.
To re-fetch the lists in the Email Optin module, you will need to go to that Module's Settings > Content Tab > Email Account settings group and then click on the Fetch Lists button.
If you're using the Bloom plugin, you will need to go to Bloom > Email Accounts > Accounts Icon and then click on the refresh icon to re-fetch the lists.
Address field problems:
Another common problem is having the Address field as required, in the list's settings.
By default, MailChimp only allows that field to be filled from their hosted forms, it's not possible to send data through that field from 3rd party plugins such as Divi Builder or Bloom.
You can adjust it on the MailChimp website and make sure it's disabled:
If you are still having issues, then it's best to see the error message(s) and reach out to us for a more in-depth investigation.
You can see the error message(s) by going to your MailChimp Account page:
And then click on Extras > API keys:
You should now see any failed calls along with the error message(s) at the bottom of that page: