When i viewed an external list in SharePoint 2013, i got the following error message.
Error Message : Message from External System : ‘The username returned by the Secure Store Provider is not of the form ‘Domain\User’.
Resolution : In this case, the error occurred because i didn’t give domain name in secure store target application.
Central Admin -> Application Management -> Manage Service Applications ->Secure Store Service -> click on target application ID -> select “Set Credentials” -> Provide domain name
SharePoint Exploration 