After applying Windows Updates for Windows Server 2008 R2 in early 2018, the virus definitions for Symantec Mail Security for Microsoft Exchange (SMSMSE) no longer update at the scheduled time. If LiveUpdate is run manually, the definitions update successfully.

When opening the Windows Task Scheduler, an error is displayed "Task AtX: The task XML contains an unexpected node".

When installed to Windows Server 2008 R2 and before, SMSMSE creates scheduled tasks for both LiveUpdate and scheduled reporting using the Windows Task Scheduler 1.0 API. These tasks will show a format AtX in the task scheduler, where X is a number.
Some time in January 2018, a Windows update appears to have disabled the ability to run At type scheduled tasks. Symantec is investigating which patch made this change and will update this article when more information is available.

This issue is resolved in Symantec Mail Security for Microsoft Exchange 7.9.0

Work-around:

Symantec has released a patch to allow administrators to specify to use the 2.0 version of the Task Scheduler API for SMSMSE's scheduled tasks.
 

Note: This patch applies only to SMSMSE version 7.5.6. Upgrade to 7.5.6 prior to applying this patch.

To switch scheduled tasks to version 2.0:

1. Download the attached file 4106608.zip.
2. Unzip the contents of the file to a convenient location.
3. Open the SMSMSE console, and navigate to Admin -> LiveUpdate/RapidRelease schedule.
4. Uncheck "Enable automatic virus definition updates" then click Deploy Changes.
5. Stop the Symantec Mail Security for Microsoft Exchange and Symantec Mail Security Utility services.
6. Take a backup of the file SAVFMSEShared.dll from the C:\Program Files (x86)\Symantec\SMSMSE\7.5\Server directory.
7. Copy the SAVFMSEShared.dll from the attached hotfix to C:\Program Files (x86)\Symantec\SMSMSE\7.5\Server directory.
8. Open the registry editor.
9. Navigate to HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Symantec\SMSMSE\7.5\Server.
10. Right click and select New -> DWORD (32-bit) value.
11. Name the new DWORD value UseTaskScheduler20Interfaces then set the value to 1.
12. Start the Symantec Mail Security for Microsoft Exchange and Symantec Mail Security Utility services.
13. Open the SMSMSE console, and navigate to Admin -> LiveUpdate/RapidRelease schedule.
14. Check "Enable automatic virus definition updates" then click Deploy Changes.

After completing these steps, SMSMSE should now update definitions at its scheduled time(s).