Let's return and check this:
1. You have only 1 Task Server which is running on your ITMS server or there is a remote Task Server as well?
2. What is the version of Task Server(s) you have?
3. Does "Altiris Client Task Data Loader" and "Altiris Object Host Service" are running on server(s), where Task Server is installed? (You can try to restart them and try to run task again on client computer)
4. What is a Win OS on client computer, where task execution fails?
5. There is no network problems, like closed ports, etc?
6. What is the version of Client Task Agent plug-in in SMA on your client computer, where task fails?
- You can check version in properties of SMA. Mouse double click on SMA tray icon:
7. You've tried to run similar "Run Script" task?
8. What says SMA in logs, when task execution fails?
- Logs on client computer are here ⇒ C:\ProgramData\Symantec\Symantec Agent\Logs
9. Make sure that there is no crash occurs of SMA on client computer
- Crash dumps of SMA are here ⇒ C:\ProgramData\Symantec\Symantec Agent\CrashDumps
10. Did you try to execute "Run Script" task, for example using another client computer or even for local ITMS server? What is result?