Spotfire Statistics Services: SAS home not a valid directory error even after setting the correct SAS home

When attempting to send a Spotfire data function to a TIBCO Spotfire Statistics Services (TSSS) instance that has been configured to use SAS as its statistical engine, the following error is returned even though -Dspserver.sashome is set properly in Java Properties.:

Could not execute function call.
TIBCO Spotfire Statistics Services returned an error: 'SAS home not a valid directory'.
   at Spotfire.Dxp.Data.DataFunctions.Executors.RemoteFunctionClient.OnExecuting(FunctionClient funcClient)
   at Spotfire.Dxp.Data.DataFunctions.Executors.AbstractFunctionClient.<RunFunction>d__0.MoveNext()
   at Spotfire.Dxp.Data.DataFunctions.Executors.SPlusFunctionExecutor.<ExecuteFunction>d__3.MoveNext()
   at Spotfire.Dxp.Data.DataFunctions.DataFunctionExecutorService.<ExecuteFunction>d__6.MoveNext()

This is a misleading error message. The problem is due to Windows permissions settings. 

In Windows, on the TSSS server, open the Control Panel and then use it to open the Services panel. 

Find the Service for TSSS, right-click on it and select "Properties". Click on the "Log On" tab and inspect the account settings. 

The Service for TSSS can be set up as a Computer Account, but for SAS it must be configured for a specific user who has access/permission to run code in SAS. 

The following is from the TSSS Installation manual:
 

On Microsoft Windows®, Spotfire Statistics Services normally would run as a Windows service. The properties window for a service contains a Log On tab, specifying how the service process should be executed; either Log in as:/Local System account, or Log in as:/This account. Use Log in as:/This account for the service if the time to start up a SAS software task needs to be minimized.

  Change this setting from a System Account to a specific user with correct permissions, then save these settings.  

Stop and restart the TSSS service to enable these changes.