Please try below P_K_ Comments
1. Stop Symantec Endpoint Protection Manager Service
2. Stop SMC ( Start => Run => smc -stop )
3. Delete old content in C:\Program Files\Symantec\Symantec Endpoint Protection\Shared Update folder
4. Restart Symantec Endpoint Protection Manager Service
5. Restart SMC ( Start => Run => smc -start )
https://www-secure.symantec.com/connect/forums/sharedupdates