Yes, I’m fairly sure that’s where the problem is. Anything in your error logs?
Luckily once this is fixed if you simply edit any non-synced posts and save them they will be synced Unfortunately the topic time will be whatever time it is when you first saved that post.
Grab 2.2, not only might it make things work for you but it’s always safer to upgrade anyway. I don’t think that the 2.1 branch will be maintained, and there’s a very serious XML-RPC issue in 2.1.2 that leaves you very vulnerable (note 2.1.3 isn’t worth upgrading to as I don’t think it’ll get more fixes).