Red Hat DIRECTORY SERVER 7.1 - ADMINISTRATOR Administrator's Manual page 570

Troubleshooting
Error 5 NT4 and Active Directory: After synchronization, the status shows error 81.
One of the sync peer servers has not been properly configured for SSL
communication. Examine the Directory Server access log file to see if the
connection attempt was received by the Directory Server. You may also find
helpful messages in the Directory Server's error log file.
To narrow down the source of the misconfiguration, try to establish an LDAPS
connection to the Directory Server. If this connection attempt fails, check all
values (port number, hostname, search base, and so forth) to see if any of these are
the problem. If all else fails, reconfigure the Directory Server with a new
certificate.
If the LDAPS connection is successful, it is likely that the misconfiguration is on
the Windows server. Check that you have properly configured the NT4 LDAP
Service and Password Sync for SSL and that the NT4 LDAP Service is running.
Examine the Windows event log file for error messages.
NOTE
570 Red Hat Directory Server Administrator's Guide • May 2005
A common problem is to fail to trust either your certificate authority
when configuring Windows Sync service certificates or the Active
Directory or NT4 Server certificate authority in the Directory Server.