Mac (Server 10.8.3) DHCP behaving very odd

Hello!

I have recently figured out a way, which turned out to be very important fo rme. Since I am living in a dorm house, my internet is limited to only one registered MAC-Address. But since I own a Mac Mini, a LAN-cable and an AirPort Express (latest as of writing), it made *click* and I quickly set up the Mac Mini to be the registered system, made a custom pfctl config (since the default internet sharing totally didn't work 😟) and even wrote some scripts to automatically change the shared interface, in case I was using a broadband dongle, which I am actually doing at times.

The system works just perfectly...well, only for apple devices.

As soon as a non-apple device tries to join the network, they're erroring. Using ipconfig /renew shows "The data is invaild."

Reviewing the setup, I realize I have actually 2 DHCP servers - one being the router with the registered MAC-Addresses, and the other being my own Mac Mini. The AirPort only works in Bridge-Mode.

By looking for suggestions, I was told to check if i have a network group - i have one, called "workgroup" which apepars to be the default one being used in larger windows networks. But although I have that, I still have no functional network on my Windows - but for sure on all my iDevices and MacBook Air.

Does anybody know how I can solve this issue? It's quite annoying typing in manual IP's over and over -.-

Regards, Ingwie