MacOS – Is this network blocking the VPN

I use a VPN from my Mac (OS X El Capitan). It works fine everywhere… except for one particular guest Wifi network, where I simply cannot connect to the VPN: it shows "Connecting…" in the menubar for a few seconds, then fails. Verify the server address and try reconnecting.". The VPN configuration seems fine, as it works fine from every other network.

How do I debug the cause of this? Is this network blocking access to this VPN? How would I tell?

Here are the log messages in /var/log/system.log:

racoon[40908]: accepted connection on vpn control socket.
racoon[40908]: IPSec connecting to server X.X.X.X
racoon[40908]: Connecting.
racoon[40908]: IPSec Phase 1 started (Initiated by me).
racoon[40908]: IKE Packet: transmit success. (Initiator, Aggressive-Mode message 1).
racoon[40908]: >>>>> phase change status = Phase 1 started by us
racoon[40908]: IKE Packet: transmit success. (Phase 1 Retransmit).
--- last message repeated 2 times ---
racoon[40908]: IPSec connecting to server X.X.X.X
racoon[40908]: Connecting.
racoon[40908]: IPSec Phase 1 started (Initiated by me).
racoon[40908]: IKE Packet: transmit success. (Initiator, Aggressive-Mode message 1).
racoon[40908]: >>>>> phase change status = Phase 1 started by us
racoon[40908]: port 62465 expected, but 0
racoon[40908]: IKEv1 Phase 1 AUTH: success. (Initiator, Aggressive-Mode Message 2).
racoon[40908]: >>>>> phase change status = Phase 1 started by peer
racoon[40908]: IKE Packet: receive success. (Initiator, Aggressive-Mode message 2).
racoon[40908]: IKEv1 Phase 1 Initiator: success. (Initiator, Aggressive-Mode).
racoon[40908]: IKE Packet: transmit success. (Initiator, Aggressive-Mode message 3).
racoon[40908]: IPSec Phase 1 established (Initiated by me).
racoon[40908]: IPSec disconnecting from server X.X.X.X
racoon[40908]: IKE Packet: transmit success. (Information message).
racoon[40908]: IKEv1 Information-Notice: transmit success. (Delete ISAKMP-SA).
racoon[40908]: failed to send vpn_control message: Broken pipe
racoon[40908]: glob found no matches for path "/var/run/racoon/*.conf"
racoon[40908]: IPSec disconnecting from server X.X.X.X

There is a precisely 30-second gap between Phase 1 established and IPSec disconnecting. I have full network connectivity from this guest Wifi network as long as I don't enable the VPN. I am in the US, so it is unlikely this is government censorship, but it might be some kind of blocking by the maintainer of the guest network. I can successfully ping the server IP address (anonymized to X.X.X.X in the logs above). Anything else to check?