Thank you, Grand Sorcerer
Your answer makes a lot more sense than what I received from Kobo. Indeed, I do have wifi problems at home, where it occurred, and the connection is a bit instable. Still, I believe the Kobo software should be able to handle that ... if they were willing to debug it when users run into problems.
Regarding the strange "offline reading limits", here is what I found about it. It is a google entry for Kobo page that no longer exists.
I called Google with:
site:kobo.com sync offline reading and it returned this first answer:
kobo.com/help - Reading Kobo Plus books while offline
https ://stage.kobo.com/help/en-US/.../reading-kobo-books-while-offline
Once you've opened 15 Kobo Plus books or reached the end of your 30-day offline
reading period, you'll need to reconnect to Wi Fi and sync your Kobo ...
But the intended page, which I guess should be at
https://stage.kobo.com/help/en-US/ar...-while-offline , no longer exists.
I do not know about these 15 books or 30 days limits. I do not even know what is a "Kobo Plus book". What does the "Plus" stand for?