Hello,
What happens if Network Threat Protection is Disabled??
It seems this is not an issue with SEP but Microsoft.
Check this Article and Download and install the Microsoft Hotfix:
When you use a VPN connection that uses Smart Card authentication on a client computer that is running Windows Vista, Windows Server 2008, Windows 7 or Windows Server 2008 R2, the computer stops responding
http://support.microsoft.com/kb/975212
Once, the machine is updated. Please restart the machine.
Hope that helps!!