@KevinH.
Actually I don't have Gatekeeper disable. Just told my Mac to open apps downloaded from any source.
I don't have multiple displays. Nor antivirus/mailware running.
Renaming sigil.ini (then recreating one when Sigil launches) sorts no results.
Sigil doesn't crash. I have to force quit it. Here's the crash resume
https://workupload.com/file/tJM5CC22VGw
I will try creating a new user on my system to see if I can use Sigil from there.
And big thanks for your good support.