On one of the client machine please try to delete the 'Task Cache' by cleaning up all files under the below folder...
Before deleting stop the 'Symantec Management Agent' Service.
C:\Program Files\Altiris\Altiris Agent\TaskManagement\cache\*.* (Delete all files..)
Start the agent...
Check if it works well.
If it works, i shall provide you a script which you can run on all the clients to fix the issue.