Yeah. Silly question, but you are sure that Windows security isn't blocking internet access for Steam? And Steam is set OK? And you've tried again on another day to be sure it's not just a Steam severs issue? (They can be flaky.)
Personally, I disabled cloud sync in the game's Steam properties, because it takes sooo long (with all my snapshots) and undeletes snaps I cleared out while the game wasn't running, etc.