Jim,
Thanks for your time and reply note.
After a few tries, i got it: you must CREATE a new user/password in the Media Network options for the Authentication - Library Server Sync line.
Once you create this User/Pass, and configure it equally on JRemote (or any other external access i would guess), you are OK.
This could be better explained in the box that asks for this information, because there are several other levels of user/pass in the way (the computer running JRiver Server and the NAS storing the Library in my case, both with their own authentications), and one is not sure what is the combination required here.
I would advise in the future to add a more detailed explanation for this option, for example "Create username and password for external access to allow Lib Sync". This could be more meaningful.
Cheers to all that read my message.
Hope my sharing experience may help others,
Thnxs
Ray