Recently I ran in to a problem where I received inconsistent results when backing up Windows 2008 R2 servers and received the error, "V-79-57344-65072 - The connection to target system has been lost. Backup set canceled."
I opened a case with Symantec Support and worked with 6 different support technicians (including a senior member) over a 3 month period. Eventually I was able to determine the issue myself through trial and error.
It turned out that IPv6 service on the remote servers and media server was causing unpredictability in traffic routes and would cause the media server to believe that it could no longer communicate with the remote servers.
IPv6 by default is available with Windows 2008 R2 and although my enterprise networks fully support the standard, it caused BE2010 R3 SP2 to get confused about the proper path to use. Rather than disabling IPv6 on all the servers - I changed the network and security setting as follows:
Network interface: "Use any available network interface"
Protocol: IPv4
See the screen shot attachments for default settings and changed settings as a workaround.
