VPN on OS X Lion Server I'm trying to configure VPN on my Mac Mini Server running OS X Lion 10.7.1 with Server app.

I have a SKY D-Link ADSL router connected via ethernet is Apple AirPort Extreme and my other devices including my Mini connect to my AirPort Extreme via Wi-Fi.

I don't have a Static IP so I am using DynDNS. I have installed the DynDNS Mac application on my Mini.

I have set the hostname on my mini to the hostname provided by DynDNS and I have set my Mini a static LAN IP of 192.168.0.100.

I have turned the VPN service on the Mini's server app, set a secret key and set a range for VPN connections of 192.168.0.101 - 192.168.0.200

For testing purposes I have enabled and forwarded all ports for incoming services to 192.168.0.100 on my D-Link Sky router.

I have tried to connect my iPhone using my DynDNS host name authenticating with a network account I have created and my secret key. I get an error the L2TP-VPN server did not respond.

My internet connection is working correctly on both my network and my iPhones data plan I have checked with my ISP and they do not block VPN connections.
I was getting the same error. In my case it turned out to be a conflict with "Back to My Mac"—specifically within my Airport Extreme. Once I removed my iCloud user account from the Airport Extreme, OS X Server's VPN started responding.

If there are any Airport Extreme/Express devices on your network, definitely remove any iCloud/MobileMe accounts using Airport Utility.

You may also have Back to My Mac turned on in System Preferences—and it's conceivable that could cause a problem as well...

I recommend (at least temporarily) checking that no user accounts have Back to My Mac enabled in the iCloud pane of System Preferences; I would verify this on every user account on every Mac on your network.

Good luck!