Hello,
"Thumbs Up" to Pete's Advice above.
Are you using a Proxy on the Environment??
If yes, have you made the necessary changes in Symantec Endpoint Protection Manager for the same.?
Incase, the above steps haven't resolved your issue.
Could you upload the Log.liveupdate from
C:\Documents and Settings\All Users\Application Data\Symantec\LiveUpdate
Hope that helps!!