...the LU session itself is working fine as per the below logs entries:
02-08-2012, 10:35:30 GMT -> EVENT - PRODUCT UPDATE SUCCEEDED EVENT - Update available for SEPM Virus Definitions Win32 v12.1 - MicroDefsB.CurDefs - SymAllLanguages. Update for CurDefs takes product from update 0 to 120801037. Server name - liveupdate.symantecliveupdate.com, Update file - 1343897304jtun_sep12enncur26.m26, Signer - cn=Symantec Corporation,ou=Locality - Culver City,ou=Product Group - LiveUpdate,ou=SymSignature 2005,o=Symantec Corporation, package install code 0. The Update executed with a result code of 1800, => Success
02-08-2012, 10:35:30 GMT -> EVENT - PRODUCT UPDATE SUCCEEDED EVENT - Update available for SEPM Virus Definitions Win64 (x64) v12.1 - MicroDefsB.CurDefs - SymAllLanguages. Update for CurDefs takes product from update 120715009 to 120801037. Server name - liveupdate.symantecliveupdate.com, Update file - 1343897304jtun_emt64sep12en120715009.m26, Signer - cn=Symantec Corporation,ou=Locality - Culver City,ou=Product Group - LiveUpdate,ou=SymSignature 2005,o=Symantec Corporation, package install code 0. The Update executed with a result code of 1800, => Success
Unfortunately it looks like the sesmlu.log file is from a completely different time frame, and only shows logs from 12:06 onwards (a good 1.5 hours after the above LU session), so I can't tell what's happening.
It is apparent however, that errors are getting logged in the sesmlu.log, but there's not much info on them.
You might be looking at a repair of the SEPM here I'm afraid! Before that though, can you confirm you have plenty of disk space free?