[stumped]

Thanks for reply.
Afailk I have never used any user name and password setup for calibre . And There is no password needed to access my documents as a user in windows.
And only one of my two devices running cc has that decrypt key error mention in logs
So that is a minor puzzle but maybe not a cause.
Unless I misread the log sequence that decrypt error comes before any books are requested. It is part of the initial set up of the connection ?
And it was there just once per session, of once per book request.

My own quick try to reproduce the issue failed. I edited a book again in calibre then asked for it via content server and it was served ok. It did not go missing!

I know that kovid added something recently. To content server. It is in his release notes. It says try to check if file has been edited outside of calibre and serve the latest version. I edit in sigil.. I don't know how he implemented that though so I don't know if that could be a factor ?
But because the download failed for two different books, I can't think of a cause. There is no way I would ever have had both books left open for edit or in use in another program.
The second bookwhich did not download,. A painted house, had not been recently tweaked. I tried it after the first download failed because it was the next title on screen in that category.
Then I think I tried some others which worked, so then went into stop, start cc, restart tablet etc.
Please don't some too much time on it. It may have been a one off glitch with calibre that will never get explained. It probably doesn't not help that I leave calibre running or idling for weeks, and only shut it down if there is a problem or if windows wants to upgrade. I just sleep the pc with calibre still running.
In calibre I think there were epub and original epub files for those books. So only one valid format.