Connect the Kobo to a pc, find and replace this file:
https://www.transfer.sh/5zpxv/KoboReader.sqlite
(it will be in a .Kobo subfolder, I forget exactly where.)
eject the kobo, wait for it to come up to the home screen, then restart completely, (Make sure it's powered off, not just sleeping.)
You should be good to go.
No guarantees a future update (which you have to install manually, btw), will not throw a hairball. It's happened to me once before.