No se puede conectar a la VPN a través de iOS

He configurado un servidor Mac OS X 10.8 que ejecuta el servicio VPN. Desde mis Macs puedo conectarme al servidor VPN sin problemas, pero los dispositivos iOS no pueden conectarse.

La consola muestra el siguiente mensaje al intentar conectar un iPhone:

9/8/13 11:02:51.704 AM racoon[313]: Connecting.
9/8/13 11:02:51.704 AM racoon[313]: IPSec Phase1 started (Initiated by peer).
9/8/13 11:02:51.706 AM racoon[313]: IKE Packet: receive success. (Responder, Main-Mode message 1).
9/8/13 11:02:51.706 AM racoon[313]: IKE Packet: transmit success. (Responder, Main-Mode message 2).
9/8/13 11:02:51.771 AM racoon[313]: IKE Packet: receive success. (Responder, Main-Mode message 3).
9/8/13 11:02:51.809 AM racoon[313]: IKE Packet: transmit success. (Responder, Main-Mode message 4).
9/8/13 11:02:51.852 AM racoon[313]: IKE Packet: receive failed. (Responder, Main-Mode Message 5).
9/8/13 11:02:54.570 AM racoon[313]: IKE Packet: transmit success. (Phase1 Retransmit).
9/8/13 11:02:54.692 AM racoon[313]: IKE Packet: receive failed. (Responder, Main-Mode Message 5).
9/8/13 11:02:54.854 AM racoon[313]: IKE Packet: receive failed. (Responder, Main-Mode Message 5).
9/8/13 11:02:57.572 AM racoon[313]: IKE Packet: transmit success. (Phase1 Retransmit).
9/8/13 11:02:57.661 AM racoon[313]: IKE Packet: receive failed. (Responder, Main-Mode Message 5).
9/8/13 11:02:57.857 AM racoon[313]: IKE Packet: receive failed. (Responder, Main-Mode Message 5).
9/8/13 11:03:00.575 AM racoon[313]: IKE Packet: transmit success. (Phase1 Retransmit).
9/8/13 11:03:00.645 AM racoon[313]: IKE Packet: receive failed. (Responder, Main-Mode Message 5).
9/8/13 11:03:00.859 AM racoon[313]: IKE Packet: receive failed. (Responder, Main-Mode Message 5).
9/8/13 11:03:11.282 AM com.apple.time[373]: Interval maximum value is 946100000 seconds (specified value: 9223372036854775807).
9/8/13 11:03:12.586 AM racoon[313]: IKE Packet: transmit success. (Phase1 Retransmit).
9/8/13 11:03:12.611 AM racoon[313]: IKE Packet: receive failed. (Responder, Main-Mode Message 5).
9/8/13 11:03:12.861 AM racoon[313]: IKE Packet: receive failed. (Responder, Main-Mode Message 5).

iOS muestra el siguiente mensaje de error: "El servidor L2TP-VPN no respondió. Intente volver a conectarse. Si el problema continúa, verifique su configuración y póngase en contacto con su administrador".

¿Por qué no puedo conectar un iPhone al servidor VPN mientras que un Mac no tiene problemas de conexión?