Activision says the Call of Duty: Modern Warfare 3 and Warzone season 1 error message warning you your "data is corrupt or didn't download properly" and saying "you must rest your rank and unlocks to continue" is in fact an error itself.
Scanning the subreddits for Modern Warfare 3 and Warzone will yield a bunch of posts from players concerned about the error message, but the official Call of Duty support X (formerly Twitter) account says your rank, purchases, and unlocks are safe despite the alarming language. Your "loadouts, customization, and other settings" might have to be reset in order to play the game, however.
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