Hi,
As per
documentation, the CLIENT_CUSTOMER_ID_INVALID error is mainly triggered when you specified a client customer ID (not your OAuth2 client id) that is invalid or is not a number.
Client customer ID is always composed of digits with no alphabets or other punctuations except "-", and is in the form such as this example : 123-456-7890.
For us to investigate further, would you be able to share to us privately (using reply privately to author option) the complete SOAP request and response logs captured when error was encountered as well as the clientCustomerId used in the request? These information will help us verify the details of your request and check if there are any invalid inputs.
Regards,
Ivan
AdWords API Team