Hello,
There are multiple causes for this error, but most commonly there could be a problem with the virus definitions failing to load.
Uninstalling and reinstalling will often resolve the virus definition issue.
If that is not the case, then please contact Symantec Technical Support for further assistance.
Also, try Migrating to the Latest Version 11.0.6300 (RU6 MP3) and check if that resolves the issue.
Migrating to Symantec Endpoint Protection 11.0.6300 (RU6 MP3)
http://www.symantec.com/business/support/index?page=content&id=TECH155655
Why Migrate? Check these:
Release notes for Endpoint Protection and Network Access Control 11
http://www.symantec.com/business/support/index?page=content&id=TECH103087
About Maintaining Consistency of Software Versions throughout a SEP 11 Organization
http://www.symantec.com/business/support/index?page=content&id=TECH131660