For me anyway.
The problem i had was that my anti virus software (Clam sentinel) prevented the installation of the .exe file required for online play. I was able to play the tutorial and offline modes (i didnt actually try launching an offline / skirmish match but i was offered the lobby so im assuming that would have been possible)
Deactivate antivirus and run a file check by going to the steam library right clicking chaos reborn then properties > local files > verify intergrity of game files.
1 File found and updated
Reactivate anti virus and enjoy game.
Hope this helps, i realise now that this is very similar to the advise already given and maybe im just saying the same thing a different way.