When attempting to use the Marketing Cloud (building an app for it), we are getting an error message when using our obtained Access Token:
The security token could not be authenticated or authorized ---> A critical error has occurred and has caused an authentication failure. The error has been logged. Please try again and if the issue persists please contact customer support. ---> SOAP Authorization failed for Oauth token: [redacted]. Reason: ParseError.
As far as I can tell, we are getting correct tokens and end-points from the system, when I run it in debug mode, I can see the endpoints being called and the tokens being provided, and when these methods are called I get values successfully.
It's only when I try to do something (ie retrieve lists) do I get the error message above.
This was primarily via following the repo that had been pointed out to us by a SF employee (all examples in documenation show username/password methods, but not how to use an Access Token with SOAP): https://github.com/salesforce-marketingcloud/FuelSDK-CSharp/blob/Dot9/FuelSDK-CSharp/ET_Client.cs#L154
Any ideas would be greatly appreciated