This appears to happen when a Steam update isn't installed completely or correctly and is easily fixed.
Simply open up Steam, and click the 'Servers' button. Join a server from within the Steam menu system. This should allow it to complete the update process.
Maybe a corrupt installation file.
Try reinstall with a new installation file.
It works for me without any problem!
Edited by Ante
Technician
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Know this, my friend - the evil that you move against is the dark lord of terror. -Deckard Cain