The name of my workgroup is different from "WORKGROUP".
It was a matter that this friend solved by manually editing smb.conf
(as the graphic interface of Samba doesn't feature changing it.)

I intend to see if I can myself find a solution there for having the
Ubuntu computer access my Windows network.

It has come to me, that Ubuntu (to my knowledge) is unaware of my
workgroup password and that of the computers in it.