ALZDBA - Tuesday, February 20, 2018 2:37 AM

SOLUTION:

For DBMirroring to be able to use KERBEROS,
you need to register an SPN for the service account to the Endpoint port number ! 

( my SPN registrations were only set for the SQLInstances port number )

$DBMPortNumber = '5022'
New-SPN -FQServerName $('{0}.{1}' -f $TargetComputerName, $ADDomain.DNSRoot ) -PortNumber $DBMPortNumber -ServiceAccount $SQLInfo.ServiceAccount.ToString()