Thanks to a reply I got at forum.kde.org, I managed to disable the kontact autostart by disabling KOrganizer Notification Daemon from the system tray.
Everytime I log into KDE, Kontact starts showing contacts and popups the akonadi server self-test window, even if starting an empty session and not having it in autostart. Furthermore, explicitly adding it to the exclude list doesn't prevent it from starting each time I log in. Please notice that Akonadi self-test reports that a previous server error log has been found. This happens since I played with LDAP addressbook, that regularly works. Anyway, deleting the LDAP addressbook has no effect in...