If you are recieving the below error, you're using the (default) self-signed certificate. The below instructions will solve you problem.
From the Warning Screen (Red 'Untrusted' Window) select "Change Settings...". Here you should untick the "Block connections to untrusted servers" option. After completing this task and attempting to reconnect, you should receive a white "Untrusted VPN" window. On this one, tick the "Always trust this VPN server". This should fix your problem.