You were right again, I went in my library and as soon as nickel tried to calculate the thumbnail of the switchtokoreader png file, it closed nickel and went to the KSM menu.
I started nickel again and everything is fine.
But it seems that I still have a shelf problem, because the shelves that appeared after the restart are the ones made by calibre, and not the ones supposed to be built by the auto shelf script (launched in KMS).
I will try again and if it doesn't work, I will open a new tread...
Oh maybe a last question
Is it possible to backup and restore the internal memory (internal SD ?) on a Kobo H20, without opening it ? If yes, do you know a link ??
Anyway thank you very much for your help, when nickel refuses to start, it looks like to much to a brick, and it is really frightening !