The "RPC server is unavailable" error usually indicates that there is a communication problem between the domain controllers. This can be caused by a variety of factors, including network connectivity issues, firewall settings, and DNS misconfigurations.
Since you were able to force replication using the repadmin /syncall command, it is likely that the issue is related to DNS. When using the /AdeP parameters, repadmin only replicates Active Directory partition data, and does not use the RPC protocol to communicate with the other domain controller. However, when you run the repadmin /replsummary command, it uses RPC to query the replication status of all domain partitions, and this is likely where the issue is occurring.
To troubleshoot this issue, you can start by checking the DNS settings on both domain controllers. Make sure that each domain controller is using the other DC as its primary DNS server, and that there are no other DNS servers configured in the network adapter settings. You should also check that DNS zone transfers are allowed between the two DNS servers.
You can also use the dcdiag command to check for any other errors that may be preventing replication from working correctly. This command will run a series of tests on both domain controllers and report any errors that are found.
If you are still unable to resolve the issue, you may need to consult with a network administrator or Microsoft support to help diagnose and fix the underlying problem.
