[Off-Topic]
Heyho, ich hab gerade eine kleinen privaten Server aufgesetzt, tolle Sache von euch!

Als Frontend-Entwickler ist mir allerdings gleich etwas aufgefallen: eure Input-Felder haben zwar eine Border on :focus, allerdings wird die outline nicht entfernt (outline:none)

Grüße

I started a multiplayer game via Tunngle and a friend can join me via "Play on LAN", everything working fine.
I then created a headless server with a batch file, i can join via "Play on LAN", but my friend can't see the server.

What could cause this?

I also had this problem. This mainly results in the fact that the headless server installation also uses your local factorio folder in C:/users/(yourname)/AppData/Roaming/Factorio.

I just downloaded the zip-package instead. Hope this works for you also.