I think I found the problem because of for some models didn't show the menu.
Here there's the updated version of the nickel / koreader selection script:
https://www.dropbox.com/s/5va1vcdq0d...eader_v1.1.zip
To install the new script, simply extract it on .kobo folder.
No reinstall of the main advboot package is required.
Hope this time all works fine.
Andrea