Error 781 - Encryption Failed
Symptom: When connecting to a VPN server, you may receive this Error 781: The encryption attempt failed because no valid certificate was found.
Resolution: 1. If your VPN client is trying to use L2TP/IPSec for the VPN connection, you may receive above message. What you need to do is switching to a PPTP VPN connection, which doesn't need a certificate. To do this, open the properties of the VPN connection, choose the Networking tab, and change the "Type of VPN" to PPTP VPN (default is Automatic). Also make sure you have good connection. Sometimes, poor connection can cause this issue too.
2. Install a valid machine certificate on the VPN server.
3. Stop and start the IPSec Policy Agent and RRAS on the remote computer.