Hi there @AST-Redux so sorry for the delayed response on this thread! I do see we've chatted before about this authentication error in another thread. For anyone here that sees an error on PC regarding 'authenticating ownership of this product' please doublecheck that you are logged into the original Ubisoft account that activated your game in your Ubisoft Connect client. The authentication error is a different sort of issue than content being missing on the correct accounts.
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