Please show us all text in Details tab of failed job.
Have you tried bptestbpcd from master server and media server?
Which OS on master? On media server? And client?
Which NBU version on master? media? client?
I am surprised to see connection on port 13724 (vnetd).
As from NBU 7.0.1, NBU will try to connect to port 1556 (pbx).
Any firewall in place?
Or OS firewall on client?
Before next backup attempt or bptestbpcd from media server, please create bpcd log folder on the client.
This log should tell us what happened on the client and the reason for status 21.