Hello Simon,
As stated in the Article provided above by Thomas, the Possible causes of this issue is due to Corrupt definitions.
However, as stated by you this happens only on the SEPM server machine.
I personally would request you to :
1) Uninstall Symantec Endpoint Protection Client along with Liveupdate from the machine.
2) Delete all the Leftover files of Liveupdate like:
On Windows 2000, XP and 2003:
- C:\Documents and Settings\All Users\Application Data\Symantec\LiveUpdate
- C:\Program Files\Symantec\LiveUpdate
On Windows Vista, 7 and 2008:
- C:\ProgramData\Symantec\LiveUpdate\
- C:\Users\All Users\Application Data\Symantec\LiveUpdate
3) Install the Latest version of Endpoint Protection Client (11.0.6300)
Follow the Article to Migrate to the Latest version :
Migrating to Symantec Endpoint Protection 11.0.6300 (RU6 MP3)
http://www.symantec.com/business/support/index?page=content&id=TECH155655
I am sure that would work for you.