General :: VPN With Mac 10.8 OS X Server?

I have a Mac mini running OS X 10.8.2 with the OS X Server 2.2.1 from the app store, and I have set up the VPN using L2TP in the Server.app interface. I have tested this VPN connection using a Macbook, which works, but I can't figure out how to get Android's built-in VPN to work.

Current set-up:

I have opened ports 500, 1701, 1723, and 4500 on my router.

I am using a dynamic DNS from [URL], we'll say [URL]

I have set a "Shared Secret", we'll say 1234567890

I have set up an account for my android phone on the server, let's say the user name is "nexus" and the password is "google" On the Macbook, I simply use the DNS, the secret, and credentials that I have set up on the server, and it connects.

On my Android device (Nexus 4 4.2.2) I am using the following settings:

Name: Mac Server
Type: L2TP/IPSec PSK
Server address: hostname.no-ip.org
LT2TP secret: (not used)
IPSec identifier: (not used)
IPSec pre-shared key: 1234567890

When I try to connect using these settings, it prompts for the username and password, so I enter "nexus" and "google". It sits there saying "Connecting..." for maybe 30 seconds and then it just goes back to "Disconnected" with no error or other message. I have also tried putting the "Shared Secret" in the L2TP secret field, but with the same result.

Is the built-in Android VPN simply incompatible with OS X Server's VPN? Or have I misconfigured something?

Note: I would strongly prefer to continue using L2TP, and not the less-secure PPTP VPN