Problem seems to be fixed with v0.12.4 when we just don't use peer-to-peer connection. The server is handling all the stuff pretty good, without lagging at all and with a really good upload speed!

I already posted this in the Multiplayer section, but I think that's not the right area to do this. So I will give it a second try here. Maybe there's a mod to delete or merge my thread from the multiplayer section:
https://forums.factorio.com/forum/viewtopic.php?f=53&t=14600

Hey folks,
I played ...

Hey folks,
I played around a little bit to set up a dedicated Server (v0.12.2). So far it's wotking fine, I was able to start the server on a VServer running a Ubuntu 14.04. Two friends of mine and me were connecting to the server, but sometimes the map upload was performed by one of our clients and ...