I just tried it now again, and now apparently Steam is downloading the required files properly. I'd wager to guess that this was an issue with the Steam client simply not replacing missing files automatically for some of us. So all appears well!
It's either:
FileKey1=%ProgramFiles%\Steam\Appcache\httpcache|*.*|RECURSE
FileKey2=%ProgramFiles%\Steam\Config\htmlcache|*.*|RECURSE
I once deleted those files my self manually and had the exact same problem. I had to reinstall Steam for it to fix its self as it wasn't redownloading them automatically.