We will perform our regular refresh of the API Sandbox database on Friday, January 26 at approximately 3pm US Pacific Time. Therefore, the Sandbox will be unavailable for a few hours.
As a result of this database refresh, all Sandbox user and account data will be erased. Therefore, you will need to first make a request without including a clientEmail header (call getClientAccounts for example) to create your five Sandbox client accounts.
-- Jon Diorio, Product Marketing