DCPROMO RPC error

I am looking to promote a domain controller on 2008 in a different site to my domain. The only existing current DCs are in another site. The sites are connected by permanent VPN, and I know its working since I am able to log onto the domain properly from any computer at the remote site, as well as join the domain as a member server from the computer I am trying to promote. But if I am running DCPromo, then it will start the process but afterwards stop with the below error message:

The operation failed because:

Active Directory Domain Services could not create the NTDS Settings object for this Active Directory Domain Controller CN=NTDS Settings,CN=ES-SERVER2,CN=SERVERS,CN=ELEMENTARY,CN=SITES,CN=CONFIGURATION,DC=stghs,DC=net on the remote AD DC hs-server2.stghs.net. Ensure the provided network credentials have sufficient permissions.

"The RPC Server is unavailable."

You can try to force Kerberos to use TCP instead of UDP in Windows, just follow the method given on this website - http://support.microsoft.com/?kbid=244474

Have you tried to upgrade the schema on the Schema master of your domain before or so?

Yeah, you are right, I have tried to do that with the forest and the domain.

Can you tell us whether the time is in sync between the sites? Also are there any multiple NICs if you have any member server with you?