Hi Tomasz.
Thanks for you answer, and for your link. Ive already found a page which gave that solution, but without explaining about password setting in sharing.
The information message in the absence of a password is already unclear ( if untrusted enter encryption password ), but its translation on the French version is really very unfortunate ( Entrez un mot de passe pour chiffrer ==> Enter a password to encrypt )
As soon as I’d removed the passwords, the shares started to synchronise normally, and now the laptop is up to date like all the others.
Thanks again.