This morning I backup some stuff before trying to restore an older install. I then decided to look if KDE's kmail would help for a while.
The MX-installer ran a lot of things and I noticed a "setting up trinity libgpg... " line. Sure enough, now TDE's kmail is running again.
So most probably something whent wrong yesterday that was set up right back today. That's where I love Debian based distributions.
Anyway, problem solved (even if I don't really understand how :)
Thierry