That usually means that the database on the device is corrupt. You can try restarting the device. That can help, but, it usually doesn't. Unless you have a recent backup of the database, you will need to do sign-out of your Kobo account on the device, and then sign back in. This will recreate the databases and reimport the sideloaded books. Unfortunately, you will lose the reading status of the sideloaded books.
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