You can't fix the backup, if you still have the old iPhone erase the corrupt backup from your computer and backup again. Or backup to iCloud.
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Same thing happened to me. I updated iOS to 11.1 on the new X and then the X accepted the backup from a 6S. I did not delete the other backup, or re-backup the 6S. This was not a seamless transfer by any means, and I have had just about every iteration of the iPhone from the first one.
Question: Back up old iphone up to date sofware to new x