Error "The RPC server is unavailable" when attempting to backup a Domain Controller (4330222)

"Unable to execute request (121).  A communication error has occurred.  The RPC server is unavailable"

Verify that DNS has been properly setup on the both the Recovery Manager for AD system and the Domain Controller.

On the Recovery Manager for AD system;
1. Open a Command prompt.
2. ping the NETBIOS name of the Domain Controller to get the IP address
3. ping -a <ipaddress> using the address from step 2.

Verify that the FQDN of the Domain Controller is being resolved to what is expected. If it is not what is expected, then verify the DNS settings for proper naming resolution.

If the backup is being stored on the RMAD server, change the computer configuration to store the backup on the DC. If the the backup completes successfully when storing the backup on the DC, the issue is most likely due to network connectivity issues between the RMAD server and the DC.