If nbu 8.1 is successfully installed on the client machine could you run these 2 commands on the client
1. nbcertcmd -getCAcertificate
2. nbcertcmd -getCertificate
If both run successfully then try the below command from the master server
“bptestbpcd -client <client_name> -verbose
If the bptestbpcd command is successful then try a backup for the client