I faced with such error
Request must contain either a valid (registered) AWS access key ID or X.509 certificate.
While trying to register an user via Cognito in DynamoDB Local. An user registers ok when I disconnect from DynamoDB and I can create an record in db programmatically.
The error comes from the DynamoDB. Please have a look: http://docs.aws.amazon.com/amazondynamodb/latest/APIReference/CommonErrors.html