Originally Posted by robko
Sounds like the database on your device is messed up. Did you unplug it without "ejecting" it from your computer by chance? That can mess up your database, or sometimes a bad epub can too. Easiest solution is to log out and back into your Kobo account ON THE DEVICE itself. This will cause it to rebuild the database, but also means any Kobo content will need to be redownloaded to the device and no guarantees on annotations etc.
I don't believe I failed to eject it. I normally eject it via calibre, rather than using the PC to do so.
I'll try your recommendation and see what happens.
ADDED: Where is 'Log out' on the device? (Do you mean 'Sign out' on the device and then 'Sign in'?)