Thank you very much for the good explanation.
There is however a small glitch in the roundcube / login that is going to be fixed. (see http://community.nethserver.org/t/roundcube-issues-connected-to-ldap/4947/14
However, I have no idea why editing the filters in the roundcube is unreliable.
Sometime the (+) button don’t work only after a refresh. The same also if I select an existing rule. I suspect the Safari browser… Firefox seems fine… at least worked with the first test…