You may want to consider opening a support case for a more rapid resolution:(http://www.symantec.com/business/support/index?page=cdlogin )
Or try this: kick off the backup manually. If it proceeds without error, cancel it, then immediately edit the backup job to run 1 minute from now, save, and let it automatically kicks off. If it proceeds, a network issue at the previously scheduled time is occuring. Sometimes simply adjusting the time to be a few minutes earlier or later resolves this). Also, check the NIC settings to ensure it isn't going into powersave mode.
If it fails, capture that same logs and attach it. Also let me know what time you set it to kick off so I can match it to the log.
(Another test is to connect a USB drive. Edit the job to point to that drive as the destination and let the scheduled backup occur.)