Hey Butzen,
you have to use the steam ID instead of the Username. This works perfect for me as additional attribute in the server.bat:
bin\AvorionServer.exe --galaxy-name avoriongalaxy2 --admin 76561197967416205 --max-players 20 --public 1 --use-steam-networking 1 --max-logs 10 pause
You can get your steam id at this website http://steamidfinder.com/ or after the first login to your server (in the logfiles)