Partage de dossier SMB avec Seven = demande login / mot de passe

That's great.

Windows only allows one set of credentials per network device (though it treats the IP address and the hostname as two different devices).  Usually your problem is because the PC already has an open network session with the NAS that uses a different set of credentials.  The net use * /delete command terminates those sessions.  Another aspect is that if the PC doesn't have saved NAS credentials, it will begin by presenting the PC logon credentials.  If those are accepted (and they likely will be if guest access is enable on the share) then those credentials are used.  That can provoke your problem later on (when you need different credentials to access a different share).

So I do recommend that you either save the NAS user credentials in the Windows credential store for each PC, or that you match the Windows username/password on your NAS account setup.  If you want the option to use a different credential, you can take advantage of the fact that Windows treats the IP address and the hostname as different machines (so they can use different credentials simultaneously).  So you can access the NAS via SMB by using a user account (with its hostname) and by the admin account (with its IP address) at the same time.