"An Error occurred trying to open the cluster at OADB"------Urgne

Dear Experts:

I have a strange Windows 2003 Cluster problem and I am running out of ideas.
I have the detailed problem descriptions here, any suggestions on this topic
are highly appreciated.

Environment:

OAPDC (Windows 2003, Domain Controller)
OADB1 (Windows 2003, SQL 2005, node 1 of the SQL Cluster)
OADB2 (Windows 2003, SQL 2005, node 2 of the SQL Cluster)

The name of the SQL cluster is OADB.Domain.com

Problem Description:

When I try to connect to OADB or OADB.Domain.com with Cluster Administrator,
the error message is:

An Error occurred trying to open the cluster at OADB
The RPC Server is unavailable
Error ID: 1722 (000006ba)

The cluster seems to be working fine. I can connect to the OADB by
specifying a "." character in the Connect to field of Cluster Administrator.
All the resources are online. A reboot can resolved the problem temporally
but this happens again after about one month.

Troubleshooting steps I have tried:

1. Confirmed the SQL Cluster is working.
2. Checked System logs and Application logs, cluster.log, there is no
related error message.
3. In the Services console, confirmed the Cluster Service, PRC Service and
Remote Registry service are started on both nodes.
4. Confirmed the OADB.Domain.com can be resolved by DNS server and I can
ping it successfully.
5. Collected dcdiag and netdiag logs on both nodes, there is no error.
6. Checked the following KBs

RPC Unavailable Error appears when you try to access a cluster Server
http://support.microsoft.com/kb/258518

Error 1722 when starting Cluster Administrator
http://support.microsoft.com/kb/228480

I am looking forward to hearing from your response. Any suggestion on this
issue is highly appreciated.

Best Regards

Robert Li