AD Replication problem

I got this error running DCDIAG.EXE

Testing server: Default-First-Site-Name\DC1
Starting test: Replications
[Replications Check,DC1] A recent replication attempt failed:
From DC2 to DC1
Naming Context: CN=Schema,CN=Configuration,DC=DigiLab,DC=com
The replication generated an error (1722):
The RPC server is unavailable.
The failure occurred at 2002-01-18 13:48.25.
The last success occurred at 2001-12-03 12:59.56.
1106 failures have occurred since the last success.
[DC2] DsBind() failed with error 1722,
The RPC server is unavailable..
The source remains down. Please check the machine.

Any suggestions? Simple and recursive DNS tests are OK.

Rob