SYMPTOM / ERROR MESSAGE: 

Error 781: The encryption attempt failed because no valid certificate was found.

SITUATION:

This occurs after clicking on a VPN client shortcut icon in an attept to reach a remote network.

CAUSE: 

The VPN client setup has become corrupt.

SOLUTION:

1) delete the VPN shortcut on your desktop.
2) start / settings / network connections 

delete the VPN icon here also. [make sure that you copy the details of how it's configured FIRST!!!!!]

3) recreate the VPN client & don't forget to tell it to add a shortcut to your desktop while doing so.