Hi M8,
As you will no doubt know these seems to be a recurring theme, a google search for this error will reveal various forum including Steam (
www.steampowered.com)
I even have a post on this (
http://forums.steampowered.com/forum...hreadid=388244)
Personally I think one of the recent security (VAC) updates has caused this as it seems commons with people working via LAN's or Routers.
Some have suggested that the use of the "sv_lan 0" command can help with some suggesting using it in the consoles and others within the listernerserver.cfg file.
I have tried both and have not been able to run an internet server from within a game. We currently use a decicate server (but even this has been hassle - see my thread).
Sorry to be all doom and gloom, but no one has been able to provide a solution which is effective
