Ken
This is to report
success.
I suppressed the password on the OPDS calibre-server. This had been declared on Koreader under a new name. I failed to modify the address using the GUI. The
modify button was stuck and gave no result.
I opened the file
settings.reader.lua, found under the "opds servers" tag, the title and the former
non-working url of this server.
I modified manually the url to :
http://192.168.....:8080/opds.
EDIT: Resuming my tries, I managed to modify both my new custom OPDS link and the local Calibre OPDS (both with the same values) and to connect each of them to the OPDS content-server of Calibre (but without password).
After that, I could connect to the Calibre server of my computer, and download a book on /mnt/onboard.
Thank you again for your patience and kindness and all your tips. Koreader is a wonderful tool.