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Can you download the client all over again? A corrupt mpq file could cause something like this.

Either you have the game running, or the bittorrent client running, or the exe file is marked as read only (right click -> properties)

That's very weird.

First, with LAA applied, this shouldn't happen at all.

Second, an unmodified client shouldn't need LAA to work correctly.

Have you tried downloading the client again?

Can you show the exact error?