I know this is a super old issue but I was running into the same problem. The v3 URL is different and is below: https://idfed.constantcontact.com/as/token.oauth2 Take note to the URL above not ending in a forward slash. My issue was that I was using a forward slash: https://idfed.constantcontact.com/as/token.oauth2/ The above URL was giving me a 404 error because of the forward slash. Be wary!
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