When launching the SIM (Symantec Installation Manager) it fails to configure and load with the following errors:
If the customer pastes that URL into the browser he can access the product listing .xml file.
Customer was using an incorrect user name for the proxy settings when requested by the SIM installer.
The customer was using "domain\user name" in the "User Name" field in the proxy configuration page.
Had to delete the domain and only use the "user name" and the SIM installer could then access the product download .xml file and finish installing.