The database on your Kobo is corrupt.
If you have a database backup taken with Kobo Utilities, restore from the last good one.
Otherwise sign out of your account and sign back in. That will fix the database but your reading stats and positions will get wiped.
If you're using firmware 4.31, there's been reports of it being a bit unstable. Several of us have downgraded to 4.30. Ask in the
Kobo forums for more info.