VPN Error Code: 789, 835
789: The L2TP connection attempt failed because the security layer encountered a processing error during initial negotiations with the remote computer.
835: The L2TP connection attempt failed because the security layer could not authenticate the remote computer. This could be because one or more fields of the certificate presented by the remote server could not be validated as belonging to the target destination.
Possible Causes: This is a generic error which is thrown when the IPSec negotiation fails for L2TP/IPSec connections.
Possible causes for this issue could be:
a> L2TP based VPN client (or VPN server) is behind NAT.
b> Wrong certificate or pre-shared key is set on the VPN server or client
Possible Solution: Make sure Pre Shared Key (PSK) is used and correct PSK is configured on the client.