Hello user158539,
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.
Based on our system logs for the API key that has most recently connected with the account that you are posting from, I found quite a few responses which indicated that the authorization code might not be getting exchanged for the first Access Token and Refresh Token set within the 60 second timeframe before it expires.
When going through the V3 API OAuth Server Flow, the Authorization Code from steps 2 and 3 only has a lifespan of 60 seconds, and needs to be exchanged for your first token set within that timeframe or you will need to generate another. Until you get your first set of tokens to authenticate the connection, you will not be able to make successful calls to the API.
V3 API OAuth2.0 Server Flow
https://v3.developer.constantcontact.com/api_guide/server_flow.html
If you feel that my reply was not a good fit for your inquiry, please feel free to email our team directly at webservices@constantcontact.com and provide us with the API key you are using, the username for the Constant Contact account you are connecting with, so that we can look into the issue with you further.
Please have a look and let us know if you have any other questions!
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