We looked at the Log.Lue log file and saw the same error as reported by the client UI. LU is trying to download from liveupdate.symantecliveupdate.com.
LU retries the download six times and then issues *Failed download for file sesc$20antivirus$20client$20win64_12.1_english_livetri.zip". No other indicatons of errors.
The next entry in Log.Lue is interesting:
Adding an update to potentially available list by catalogue parser: SEPC Virus Definitions Win64 (x64) 12.1 RU2 H, MicroDefsB.Curdefs, SymAll Languages, CurDefs, 170717008.
***** Session Results *****
Total Updates Available: 1
Total Updates Succeeded: 1
Total Updates Succeeded - Reboot Req: 0
Total Updates Skipped: 0
Total Updates Failed: 0
Session result code: 0x00000000
There's nothing wrong with the client. We think it's on Symantec's end.