Confirm Email Opt-in Settings
It probably goes without saying, but if you haven't already done so, the first thing to do is confirm that your email opt-in form api key and list are correctly set
Remove Special Characters from the Field Names
Victor points out in the comments that he was able to solve the error by removing special characters from the field names.
Disable Caching on your Email Optin Form Pages
The "Configuration Error" message can be triggered by Divi when the opt-in form's "nonce" value is incorrect. A nonce is a random value generated each time the opt-in form is displayed, and is typically used for things such as preventing the form from being submitted multiple times by accident.
I found that when the page with the opt-in form is cached (i.e. generated only once, not each time a user visits), the nonce is not updated. The result is that when caching is enabled, the form fails to submit.
To confirm whether this is the issue on your site, try temporarily disabling caching. Then reload the page with the signup form and submit it.
If caching was the issue, then the form should be working again.
Many caching plugins allow you exclude certain pages from being cached. This allows you to re-enable caching across your site and selectively disable it on the pages which have an email optin form.
How to disable caching for a particular page:
- Cache Enabler plugin – you can exclude pages by entering their ids in the Cache Exclusions field of the settings page.
Make the Module Non-Global
Cesar notes in his comment that he "removed the global module, then it's worked". So if you have the email optin module set as a global module, perhaps try disabling global on the module, which you can do from the module's menu, as shown here: