Can't connect to VPN via iOS I have set up a Mac OS X 10.8 server running the VPN service. From my Macs I can connect to the VPN server without any problems, but the iOS devices are all unable to connect.

The console displays the following message while trying to connect an 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 shows the following error message:
"The L2TP-VPN server did not respond. Try reconnecting. If the problem continues, verify your settings and contact your Administrator."

Why am I unable to connect an iPhone to the VPN server while a Mac doesn't have any problems connecting?
I finally figured it out:

My shared secret was too complex, I configured a new one that's slightly shorter and has no special characters and now iOS is able to establish a VPN connection.

It turned out iOS can't handle shared secret of more than 32 characters.