Thanks for your reply.
I've already used Sigil, on this Mac, for at least a couple of years, without any issue. So Sigil ISN'T brand new on my Mac. Then, in the last month I wasn't able to use Sigil anymore (can't say if because of a new download version of Sigil, or for the security OS update my Mac had in the meanwhile).
Anyway, I delete any possible old Sigil related files, using App Cleaner & Uninstaller.
Downloaded the proper Sigil installer for my intel Mac (the Sigil.app-2.3.0-1-Mac-x86_64.txz) from
https://github.com/Sigil-Ebook/Sigil/releases/tag/2.3.0).
Unzipped with double click on the file (having read I wouldn't need the Terminal command anymore, which is a good thing, since I couldn't remember where I got that advice), after having moved the .txz file to the desktop.
Moved the unzipped app in my Application folder.
I have Gatekeeper disabled, so that I can run any app I want, downloaded from everywhere.
When launching Sigil (either by double click that by ctrl+click) I'm presented a very fast verifying the app small panel (which disappear on its own), and the usual warning of any new app. I click on Ok, and then nothing happens.
Sigil freezes, and all I've got is the spinning wheel...