User registration error message

Hello,

When a user registers on the website, I receive the following message:

Error: Array ( [curl] => [Error] => Array ( [Type] => Sender [Code] => MissingAuthenticationToken [Message] => Request is missing Authentication Token ) [RequestId] => 03a616bb-db8f-11e4-98f0-2705d0ad8582 )

Can you please help me fix this?

Thank you,
Greg