Thank you for reaching out to Constant Contact API Developer Support. My team is here to assist outside software developers with questions about building into Constant Contact's API.
If you are experiencing this issue when logged into a standard account via username and password when trying to access the My Applications page, then it is most likely a security or connectivity issue. For this, we recommend trying to access the page from a different browser, device, and/or network to narrow down the root cause.
Troubleshooting Connectivity Issues
Safelist Constant Contact web domains in a security program
You can also check https://status.constantcontact.com/ to verify whether there was a service issue at the time you were experiencing the issue.
Alternately, if your account is related to a partnership and you were attempting to create a key while impersonating into a child account under the partner tools, that is another possible cause for the error being returned. V3 API keys can only be created/connected via genuine users who are logged in via their username and password. Our impersonation code for Partners has changed recently so we could implement some security enhancements. What happens now is when impersonating, a temporary userID is created which expires after about 15 minutes.
If the issue persists, please feel free to reach out to our team directly/securely via email at firstname.lastname@example.org and reference case #30519274 to let us know what best matches your situation: which user/account you are logged into when trying to access the My Applications page, whether you are logged into a managed account via the partner console, and any additional details you are able to provide, such as the name of the integration that you were generating the key for, and whether it is 3rd party developed or created by your organization.
Please have a look and let us know if you have any other questions!