OS X Server VPN - Connecting from Windows

I am running 10.10 and the latest version of Server to power my VPN. I am able to connect to my VPN without an issue when I connect using my iPhone or iPad. However, I also have a Windows laptop that I need to use with the VPN as well. I've navigated Windows 8.1's bizarre networking settings, and I should have it all set up. I have made a connection in Control Panel --> Network and Internet --> Network Connections. I went into the Properties to configure it as Layer 2 Tunneling Protocol with IPsec, Optional Encryption and added my preshared key under the Advanced Settings. When I try to connect, it prompts me for my username and password, so I enter my info that I use to log into my Mac (just like I do on my iPhone) and it fails to connect. This is the error I get, and I'm perplexed because I can connect to my VPN as I sit in the same place, going to the same server. I know this is not a Windows forum, and FWIW I also have a cheapo Android tablet to test with as well and it fails to connect as well. I thought the OS X VPN was built on standard tech, so Apple isn't locking out non-Apple devices, are they? Anyway, here's the error I get from Windows: