Active Directory error 1168 on W2K8R2

Hi all,

I have 3 domain controlles in single domain in a single forest: 2xW2K8R2 and 1xW2K3R2. Forest functional level is Windows 2003. All FSMO roles holding one of W2K8R2 domain controllers that displays this error in period of 2-4 days:

Internal error: An Active Directory Domain Services error has occurred.
 
Additional Data
Error value (decimal):
6
Error value (hex):
6
Internal ID:
1240101

I was try to diagnose problem, but "dcdiag /q" command display no errors, and "repadmin /replsum" command also not display errors or fails in replication.

Any ideas?