Hello again,
following the advices of VS Developer community, I looked into “C:\Program Files (x86)\Microsoft Visual Studio” directory and I saw, together with the Installer folder, another folder called Installer.old.
I deleted the first one and renamed the “.old” one into “Installer” and then everything worked fine, I was able to update my VS again.
It seems that I had a corrupted version of the software but, luckily, an old one was being stored for backup purposes (I suppose).
Then I updated everything (VS Installer as well) and now everything is fine.
Thank you for your support.