Hi
Better late than never.
Sigil-git 9.5. (coming from Aur repo) has been compiled and now replaces my previous Arch 9.5. version of Sigil. Following the advices I read on screen, I also installed some "optional" dependencies, required for some plugins: python-regex, python-cssselect, python-html5lib, python-cssutils
Now "test-plugin" is 100% successful and installation of any plugin is a breeze.
Thanks for all your advices.