Logged out of Kobo desktop

Slightly unexpectedly, when I started Kobo Desktop a few hours ago it popped up a dialog saying my books were not all available to me any more. That sounded scary but as it had logged me out I think that's all that it meant.

Logging back in again was fine, but I had to download all my books from Kobo again. So ultimately no harm done. (Except I wish Kobo desktop had a "download all" option. Having to click on every single book is a chore.)

However it was pretty unusual. Has something changed in Kobo Desktop recently that might have caused this? I have probably upgraded macOS since last running it, possibly changed my billing address too, otherwise nothing really has changed. Changing billing address has not previously caused it to freak out.