I'm running Mac OS X Server 10.6, providing an L2TP VPN service. The VPN works just fine when connecting from all computers except one --- this one computer stays at the "Connecting..." stage for a while, then says "The L2TP-VPN server did not respond".

In the console, I see this:

 6/7/10 10:48:07 AM pppd[341]   pppd 2.4.2 (Apple version 412.0.10) started by jdoe, uid 503
 6/7/10 10:48:07 AM pppd[341]   L2TP connecting to server 'foo.bar.baz.edu' (256.256.256.256)...
 6/7/10 10:48:07 AM pppd[341]   IPSec connection started
 6/7/10 10:48:07 AM racoon[342] Connecting.
 6/7/10 10:48:07 AM racoon[342] IKE Packet: transmit success. (Initiator, Main-Mode message 1).
 6/7/10 10:48:08 AM racoon[342] IKE Packet: receive success. (Initiator, Main-Mode message 2).
 6/7/10 10:48:08 AM racoon[342] IKE Packet: transmit success. (Initiator, Main-Mode message 3).
 6/7/10 10:48:08 AM racoon[342] IKE Packet: receive success. (Initiator, Main-Mode message 4).
 6/7/10 10:48:08 AM racoon[342] IKE Packet: transmit success. (Initiator, Main-Mode message 5).
 6/7/10 10:48:11 AM racoon[342] IKE Packet: transmit success. (Phase1 Retransmit).
 6/7/10 10:48:14 AM racoon[342] IKE Packet: transmit success. (Phase1 Retransmit).
 6/7/10 10:48:17 AM racoon[342] IKE Packet: transmit success. (Phase1 Retransmit).

...and the "retransmit" messages continue until the error message pops up.

So far I've unsuccessfully tried:

• rebooting
• deleting the VPN profile and recreating it
• verifying the client's internet connection (it is able to reach the VPN server)
• connecting through several different networks (in case a router was blocking VPN packets)
• disabling the Mac OS X Firewall on the client
• making sure that the VPN settings exactly match those of other working computers
• running software update (the client is on 10.6.3)

Any ideas?