An interesting error we came across today with a Windows 8 Client connecting to a customer site via VPN from a remote network. The VPN was L2TP with IPSec and the initial error 809 drew us to the usual NAT-T problem.
Current Windows knowledge base guidance for this appears to work fine for Windows 8 (involves modifying the registry): Configure an L2TP/IPsec server behind a NAT-T device
However we then came across error 850, the problem details and solution was as follows:
Error 850: The Extensible Authentication Protocol type required for authentication of the remote access connection is not installed on your computer.
Open the VPN Connection Properties > Select the “Security” tab and check “Allow these protocols”, click OK