@Clem2605: That is generally a sign of a corrupt database on the device. How it got corrupted, I don't know. A common reason is not safely ejecting the device.
The solution is to restore a recent backup of the database if you have one. If you don't, you will need to sign out of the Kobo account. That will create a new database when you sign in again. Unfortunately, you will lose the reading status and annotations from sideloaded books. If that doesn't work, you might need to do a factory reset.
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