Hi,
That error indicates that the user associated with the OAuth2 access token in the request does not have access to the account in the clientCustomerId SOAP header. Two common causes for this are:
1. The OAuth2 refresh token was obtained while logged in as an MCC, but that MCC no longer has access to the client account.
2. When obtaining the OAuth2 refresh token you (or the user who went through the flow) were logged into some other Google account that does not have access to the client account. You will be able to get a refresh token (or access token) for the AdWords scope regardless of whether the user has an AdWords account since the OAuth2 flow does not check if an AdWords account exists for the user -- it merely grants access to the AdWords scope for that user.
Also, the developer token has no impact on authorization. A valid OAuth2 access token can be used to make API requests against your account using any developer token, as the developer token is not part of the OAuth2 flow.
Cheers,
Josh, AdWords API Team