When launching SIM (Symantec Installation Manager), the following popup window is displayed:

(The product listing located at http://solutionsam.com/solutions/pl/symantec_v2.pl.xml.zip could not be accessed.)

If you click 'Cancel' on the message, or use any of the other options, the following popup window is displayed:

(Symantec Installation Manager failed to initialize. See log for details).

When you look at the SIM Install logs, the following errors are present:

Entry 1:

Failed to read product listing file 'C:\PROGRA~1\Altiris\Symantec Installation Manager\InstallHistoryPl.xml'.
The '.' character, hexadecimal value 0x00, cannot be included in a name. Line 66028, position 14.
   [System.Xml.XmlException @ System.Xml]
   at System.Xml.XmlTextReaderImpl.Throw(String res, String[] args)
   at System.Xml.XmlTextReaderImpl.ParseElement()
   at System.Xml.XmlTextReaderImpl.ParseElementContent()
   at System.Xml.XmlWriter.WriteNode(XmlReader reader, Boolean defattr)
   at System.Xml.XmlReader.ReadOuterXml()
   at Symantec.Installation.Xml.Reader.ReadProductListingXml(XmlReader xmlReader, XmlReader plLangReader)
   at Symantec.Installation.Xml.Reader.ReadXml()
   at Symantec.Installation.Managers.ProductListingManager.ReadProductListingFile(String productListingFile, String productListingLanguageFile, ProcessorType filterPlatform)

Exception logged from:
   at Symantec.Installation.Logging.LogActivity.ReportException(Int32 severity, String strMessage, String category, Exception exception, String footer)
   at Symantec.Installation.Logging.LogActivity.ReportException(String strMessage, String category, Exception exception)
   at Symantec.Installation.Managers.ProductListingManager.ReadProductListingFile(String productListingFile, String productListingLanguageFile, ProcessorType filterPlatform)
   at Symantec.Installation.Managers.ProductListingManager.LoadProductListingData(String xmlFile, String xmlLangFile)
   at Symantec.Installation.Managers.ProductListingManager.PromptForPlSelection(String source, Boolean nofileretrieved)
   at Symantec.Installation.Managers.ProductListingManager.GetAndLoadProductListings(String source)
   at Symantec.Installation.Managers.ProductListingManager.LoadCurrentProductListings()
   at Symantec.Installation.Managers.ProductListingManager.LoadAndUpdateProductListings()
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Threading.ThreadHelper.ThreadStart()

-----------------------------------------------------------------------------------------------------
Date: 10/4/2016 11:36:22 AM, Tick Count: 92498875 (1.01:41:38.8750000), Size: 2.60 KB
Process: SymantecInstallationManager (5620), Thread ID: 10, Module: SymantecInstallationManager.exe
Priority: 1, Source: Symantec.Installation.Managers.ProductListingManager.ReadProductListingFile
File: C:\ProgramData\Symantec\SMP\Logs\SIM 2016-10-04 11-32-33\a.log

Entry 2:
Failed to load Install History Product Listing Xml file 'C:\Program Files\Altiris\Symantec Installation Manager\InstallHistoryPl.xml'.
-----------------------------------------------------------------------------------------------------
Date: 10/4/2016 11:36:22 AM, Tick Count: 92498875 (1.01:41:38.8750000), Size: 448 B
Process: SymantecInstallationManager (5620), Thread ID: 10, Module: SymantecInstallationManager.exe
Priority: 1, Source: Symantec.Installation.Managers.ProductListingManager.LoadProductListingData
File: C:\ProgramData\Symantec\SMP\Logs\SIM 2016-10-04 11-32-33\a.log

ITMS 7.x, 8.x
SIM 8.x

Possible corruption of the InstallHistoryPl.xml (under ...\Program Files\Altiris\Symantec Installation Manager).

After looking at this closer the InstallHistoryPl.xml had an invalid character in its content. 

In order to fix this issue:

1. There is a backup of this "InstallHistoryPl.xml" file in the same location. It is called "InstallHistoryPl.xml.bak"
2. Rename the InstallHistoryPl.xml in use (something like InstallHistoryPl.xml.old)
3. Make a copy of the backup one and rename it to "InstallHistoryPl.xml":
4. Launch SIM one more time.