RPC function call failed. The RPC server is unavailable. w/ Application-Aware Processing

Firewall & Ports

A complete list of used ports can be found here: https://helpcenter.veeam.com/docs/backup/vsphere/used_ports.html#ms_servers

When Veeam Backup & Replication connects to a VM, it creates firewall exclusions for the ports and processes it uses. Veeam Backup & Replication, however, cannot add firewall exclusions to hardware or third-party software firewalls.

The Dynamic RPC ports assigned to the temporary guest agents are the most common ports that cause this issue when using Application-Aware Processing.

Dynamic RPC port range: 49152 - 65535

Note: The NIC’s Network Location being set to Public mode will also cause this, as Public mode sets the firewall to its strictest settings.

Antivirus & Antimalware

Please review the complete list of Antivirus/Antimalware exclusions here: http://www.veeam.com/kb1999.
 
The most common solution is to exclude the C:\Windows\VeeamVSSSupport\ folder from being scanned.

VeeamVSSSupport Service Cleanup

When no job is processing the VM, connect to it and check the following things:

1. Make sure the VeeamVSSSupport service is no longer present.
   If it is, use the command ‘sc delete VeeamVSSSupport’ to remove the service.
   
2. Make sure the c:\Windows\VeeamVSSSupport folder is not present.
   If it is, delete the folder. If it fails to delete, reboot, or resolve the file lock.

Dynamic Port Configuration

While it is uncommon, it is possible to reconfigure dynamic port allocation following the steps outlined in the following Microsoft KB article. These changes will need to be made on the server to which Veeam Backup & Replication is having trouble connecting.
 
http://support.microsoft.com/kb/154596