It looks like the game code was recently updated. Among the changes there's a quick flash effect when a card dies and it then goes gray. I don't know if this has happened to other people, but there appears to be a bug when the flash happens that locks the game up. If that happens to you, reload the browser and resync your game quickly. I wound up losing two matches to time outs today before I realized what was happening.
The gray is just added animation when you knock an opponent's card... but when it freezes, then i think you need to update your Flash player, i had a similar problem before but the update fixed it right up.
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