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"When it happens, don't close the game, the game randomly picks a server to authenticate your version is correct, and Blizzard didn't update one of their authentication servers.
So just try till you get one they did update. You'll need to enter your login details each time, but it will eventually work."
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https://www.reddit.com/r/heroesofthestorm/comments/t4o277/version_mismatch/
