I have contacted your team as instructed and heard no response after weeks.
I have rebuilt the v2 API application, key and token. Tested it is working perfectly.
I can confirm that WordPress and Divi are properly installed, the whole hosting enironment configured correctly to best practice standards, and I have tested this with ALL other plugins totally deactivated (in a different cache cleared browser).
Still the error shows.
I have checked with the Divi Theme creators who swear it is an issue with Constant Contact's API, as the code they are using is working on thousands of sites just fine. They suspected I was trying to use a V1 API Key
For most of this year now the only result my customers see on the website when trying to submit a simple sign up form is "The Mashery account associated with this request is not active, or an invalid API key was used."
I have also created a V3 API Key just in case.
I cannot see any way to check if there is a legacy V1 API Key/application to be removed.
I'm sorry if there was a support request which was missed, we have had a large increase in volume recently. Based on the documentation for the Divi Email Optin Module (found here and here), the module is indeed using our v2 API, which would require a v2 API key. Have you verified that, when adding your API key and access token to the module's settings, there are no additional characters being entered? If there are any extra characters, such as spaces before or after the key or token, removing them should resolve the issue.
If there are no extra characters, or removing them does not resolve the issue, please send us an email at email@example.com with a screenshot of the module settings attached (showing the API key and access token, as entered).