I've just done a complete reset to see whether it would improve battery problems.
At first all went well, and after doing the work on the device (parameters, dictionaries, fonts, configuration) I started to upload books. First and second batch went well, with the now usual error alert with no incidence.
But plugging it once more into the computer I have Calibre problems. There was a different error message (see below), and though everything seems fine looking at the files in window explorer, and Calibre did acknowledge it (a device column opened in the Calibre library, and a device icon next to the library icon, and the mouse hovering over the icon did say the available memory was 4.5 Gb, which is correct), the column is empty, and the device looks empty from Calibre.