Chapter 11 IPsec VPN for FortiOS 5.0 : Monitoring and troubleshooting : VPN troubleshooting tips : Try enabling XAuth
  
Try enabling XAuth
If one end of an attempted VPN tunnel is using XAuth and the other end is not, the connection attempt will fail. The log messages for the attempted connection will not mention XAuth is the reason, but when connections are failing it is a good idea to ensure both ends have the same XAuth settings. If you do not know the other end’s settings enable or disable XAuth on your end to see if that is the problem.