Same issue, though it seems to be against random opponents in practice. For example, mage will load infinitely (or at least until the error pops), but hunter will be playable. If you beat the hunter, mage (or another one) might unlock, but everything else is still inaccessible.
I have had the same problem, though its changed since my first install Oct 30th, after the log-in problems were fixed (at least the game client version changed)
On Oct 30th, when I had this problem, the client would tell me, with an error message I can't recall, that it cannot start the practice game. At which point I would click, Play/Choose again - then it would quickly start a game.
Today, it just keeps producing different loading messages.
I was using a custom Priest decks to play an Expert Mage.
I am running Windows 7 Professional 64 bit.
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