After Creating Backup Domain Controller Cannot Logon as Admin.

CAUSE

A backup domain controller replicates its database from the current domain controller. The initial replication occurs after the Netlogon service is started during the first boot after installation has completed. Before replication occurs (and user account databases are in sync), the backup domain controller has the default database (which includes the Administrator account with no password). Since the initial replication can take an extended period of time (several minutes for a large domain), there is a period after the initial boot during which you can log on to the backup domain controller using the accounts in the default database, but not those accounts in the domain.