Sorry to intrude here but I'm also having a similar problem, when I try to join a game Defcon will just crash out. However I can host games perfectly ok, UNLESS I have been onto the join game screen prior, at which point it will also crash

I got the debug.txt after I loaded the game and immediately went to join one and this is what I got:
Defcon 1.6 STEAM built Jun 23 2010
Parsing archive main.dat...DONE
Found language 'english' with caption 'English' in 'data/language/english.txt'
Found language 'french' with caption 'Français' in 'data/language/french.txt'
Found language 'german' with caption 'Deutsch' in 'data/language/german.txt'
Found language 'italian' with caption 'Italiano' in 'data/language/italian.txt'
Found language 'spanish' with caption 'Español' in 'data/language/spanish.txt'
WinSock started
Client listening on port 5011
Parsing Coastline data (1776 islands) : 21ms
Parsing International data (2753 islands) : 9ms
Parsing City data (2745 cities) : 14ms
Parsing archive sounds.dat...DONE
Authentication key set : 'STEAMJ-FOOJYZ-EFJLYO-ZJSFPG-BEP'
WinSock started
Connection established to MetaServer on port 5008. Listening on Port 5009
SteamAPI_Init SUCESS
Steam Authentication Initiated
Steam Authentication : Logging On To Steam
Requesting data from MetaServer : LatestVersion
Requesting data from MetaServer : MOTD
Steam Authentication : Getting Steam Id
Steam Authentication : SteamID = 76561197995257009
Steam Authentication : Getting Steam3 Auth
Received data from MetaServer : LatestVersion
Received data from MetaServer : MOTD
Steam Authentication : Requesting Authentication
Requesting authentication of key STEAMJ-FOOJYZ-EFJLYO-ZJSFPG-BEP
Steam Authentication Accepted : FULL GAME USER
Authentication key set : 'STEAMJ-FOOJYZ-EFJLYO-ZJSFPG-BEP'
Received Authentication : STEAMJ-FOOJYZ-EFJLYO-ZJSFPG-BEP : (keyID 1318399) : Accepted
Authentication HashToken set to 591798288
Started requesting public IP:port for socket 3760
Requesting data from MetaServer : UpdateURL
Socket 3760 identified as public IP 94.0.71.210:5011
Local IP address is: 192.168.0.88
Received data from MetaServer : UpdateURL
CLIENT : Disconnected
EclRemoveWindow failed on window LOBBY
Stopped requesting public IP:port for socket 3760
EclRemoveWindow failed on window Enter IP Manually
CLIENT : Attempting connection to Server at 69.224.37.234:5010...
Requesting connection to 69.224.37.234:5010 via matchmaker