Hi Caihua,
I haven't heard of any recent issues related to this. To better check your concern, could you please confirm if the same credentials are being used from the time that it was working properly and up to this time that you encountered this error?
Could you also send to me (reply privately to author) the complete SOAP request and response logs and the email address used to generate the OAuth2 credentials?
Thanks,
Joyce, AdWords API Team