Invalid_client: The OAuth client was not found

[Jacob Booth]

I have written a Windows service in .NET which I am migrating from v201607 to v201705 and now I get an error saying 

"Failed to refresh access token.

{

  "error" : "invalid_client",

  "error_description" : "The OAuth client was not found."

}"

Can you advise?

Thanks, Jacob

[Sreelakshmi Sasidharan (AdWords API Team)]

Hi Jacob, 

This error usually occurs when the OAuth2ClientId specified is not correct. Could you check if it is valid and also that it does not have any extra spaces? Could you also try the CURL request as specified in this link and see if you are able to generate the OAuth tokens? That will help you confirm if your client Id (OAuth2ClientId) is valid or not. 

Thanks,
Sreelakshmi, AdWords API Team