Server 2012 R2 and trying to setup a L2TP VPN to the server 2012 server. I have followed many "tutorials" with the latest being:

https://www.youtube.com/watch?v=LuWGuU2LW24

The VPN server is behind a NAT firewall/router and the firewall/router has L2TP passthrough enabled. We use a preshared key on the server and client. On the server end, i do not see any connection attempt logged in event viewer. I do see on the firewall/router the connection traffic. The error on the client side (Windows 10 insider and windows 7 and 8) is The L2TP connection attempt failed because the security layer encountered a processing error during initial negotiations with the remote computer.

I know L2TP is not recommended w/preshared key but we cant be the only ones to need to use this? Can anyone offer some setup assistance?

Hi,

The blog Troubleshooting common VPN related errors has provide detailed information about this VPN error, just for your reference:
http://blogs.technet.com/b/rrasblog/archive/2009/08/12/troubleshooting-common-vpn-related-errors.aspx

Besides, because of the way in which NAT devices translate network traffic, put a server behind a NAT device may causes unexpected results. If you want to use L2TP-based VPN connections behind a NAT, IPSec NAT Traversal (NAT-T) must be implemented at both ends of the VPN connection. Therefore, it is recommended to use public IP addresses for all servers that you can connect to from the Internet. More information you may reference:
How VPN Works section - VPN and NAT
https://technet.microsoft.com/en-us/library/cc779919(v=ws.10).aspx
How to configure an L2TP/IPsec server behind a NAT-T device in Windows Vista and in Windows Server 2008
https://support.microsoft.com/en-us/kb/926179

Best Regards,
Eve Wang