How to create a Steam-independent LAN server???

[CHSubZero1983]

I've been trying these out the last days but every time I want to create a LAN server it raises an error:

Error: Server cannot connect to Steam

I've created an online server and this is working perfectly so my DVD key is valid and should be activated.

My f****** Steam (sorry but I hate this tool) is running in offline mode.
But still it wants to connect to Steam.

What can I do?
We'd like to play this game next Saturday at our LAN meeting and we don't have access to the internet there.