Can't to connect to server within local network.

Hi there, not sure where to post this since it is a more general problem, but i am using a Java based source (Moopledv rev. 120)

Well, i have been running a server for me and my friends for a while now, i have set the host in ServerConstants.java to my WAN-ip and forwarded my ports. Turns out, all my friends can connect fine using a client with my WAN-IP. I can't even connect from my own PC. Turns out that is normal, and i can connect using Microsoft Loopback adapter.

Now comes the problem: When trying to connect to the server from another PC within my local network, i can at most get to the character selection and i will receive a "can't connect to blahblah (i think it was simply can't connect to server)" I tried a lot of loopback set-ups and i tried both a client that connects to the WAN IP and a client that connects to the host pc's local IP. Nothing seems to work?

Are there any (known) solutions to this? Or is there no way to make this work?

I know it works when i just run the server locally, without open ports, and let the pc use a client that uses my local IP. But that way none of my friends can connect...

Thanks in advance for any reply!