That means the database on the device is corrupt. It generally happens because it was unplugged from the computer without being ejected.
Unfortunately, there isn't a good fix. If you happen to have a backup of the database, you can restore that and everything should be OK.
You can try logging out of the Kobo account on the device. This should create a new empty database. Then log in again and do a sync with Kobo. After that, the sideloaded books should get added after restarting the device or connecting it to the PC.
If that doesn't work, you will need to do a factory reset. That will clean everything up and install the latest firmware.