This error is usually caused by one or more Windows VSS services being disabled. To fix it, you will need to go to Start > Run, and enter "services.msc" (without quotes). Here, you will see a list of services, their description, and whether or not they are running. Make sure RPC service is up and running.
You may also check the user who starts the service (the user specified in Veeam job for VSS connection).