The Windows NT 4.0 or earlier replication checkpoint with the PDC emulator master was unsuccessful.
A full synchronization of the security accounts manager (SAM) database to domain controllers running Windows NT 4.0 and earlier might take place if the PDC emulator master role is transferred to the local domain controller before the next successful checkpoint.
The checkpoint process will be tried again in four hours.
1722 The RPC server is unavailable.
This error is usually logged when the computer holding the PDC emulator role is being rebooted or is unavailable for any other reason.
This error does however indicate that the domain functional level is still set to Windows 2000 Mixed mode.
If you are not running any more NT4 BDCs in your network, then you should raise the domain functional level to at least "Windows 2000 Native".