THANK YOU !
With that change, I got the extra piece:
Error: [Errno 2] No such file or directory: '/etc/yunohost/certs//mastodon.domain.tld-history/20200827.161353-selfsigned/ca.pem'
That file is a link, to a link, … to a file missing on my system :
/etc/yunohost/certs/yunohost.org/ca.pem
and going through the sources, I figured it’s just a renamed copy of
/usr/share/yunohost/yunohost-config/ssl/yunoCA/ca/cacert.pem
After the copy, I can now create domains without error raised !
And that’s probably related to my yunohost-admin usual update issue as well !
I don’t know how it vanished though. I probably deleted it accidentally when I was trying to figure LE certificates eons ago.
Anyway: SOLVED!
Thanks again!