Invalid_grant authenticationfailure activating a SalesForce Sandbox instance.
Cause
The system administrator account for the sandbox has a unique username ([email protected]) instead of appending the sandbox name after the main account ([email protected]).
Default Salesforce login page does not recognize the credential used
Resolution
Use the custom domain option to redirect the login page to the custom URL
There is no need to specify the Salesforce Sandbox Name field, use the unique username instead.