The problem is caused by the Steam API: Apparently it is online, but it returns an invalid/blank authentication ticket, so our server is unable to authenticate the user. The same happened yesterday around 20 hours ago, it took a few hours until Valve fixed the problem. It does not affect all games, it depends on how the authentication is done.
If the problem will not be fixed within the next hours, we will eventually shutdown our servers temporarily (in this case the game runs in offline mode and skips the authentication procedure - so it's at least possible to play in singleplayer and LAN). Unfortunately there is nothing else we can really do about that, it depends on Valve now
The next update (which is scheduled for Monday) will have a fallback for these situations anyway, so you will at least be able to start the game even if the Steam API is offline / erroneous.