The refresh token is invalid or expired. (400/BadRequest)

Hello Support Team!

We are in the process of integrating the CC v3 API with the systems of my company. A few months ago, I created a trial developer account via https://developer.constantcontact.com/. The moving pieces of the integration process are almost complete. And, everything was working as expected until last week.

That is, I have now started to get the error message “400/BadRequest{ "error": "invalid_grant", "error_description": "The refresh token is invalid or expired."}” when refreshing Access Token (with the POST endpoint https://authz.constantcontact.com/oauth2/default/v1/token (along with clinet_id, client_secret and refresh_token corresponding to the account) from Postman or other custom applications. The refresh_token is obtained from as query parameter of the Redirect URI (following https://developer.constantcontact.com/api_guide/server_flow.html).

I have spent a fair amount of time trying to resolve the issue on my own with no success. May you please then help us resolve the issue? Thanks!

FYI - I created another app code flow in my account, but the error is still there. I can use my developer account in https://developer.constantcontact.com/api_reference without an issue though. After the integration end-to-end testing is over (planned for this month), we will be using a paid account of the company.