Hello,
A common cause of this is that you're using a Production Developer
token when trying to connect to the AdWords API Sandbox environment,
which requires a different set of credentials. Could you check if
you're going against the Sandbox environment in your code? If you're
using one of our code samples, most of them do default to going
against the Sandbox environment for testing purposes.
Cheers,
-Jeff Posnick, AdWords API Team