There was a small update to the original release, but nobody reported this problem in the Steam forum
Can you start the game directly from the executable, without the Steam client running?
C:\Program Files (x86)\Steam\SteamApps\common\divine_divinity\div.exe
Did you try re-installing the game, or verifying local files?
Is updating only automatic? Can you tell Steam not to check for updates (at least until you get the game running) or check for updates without trying to start the game?