We stumbled into similar problems…
I think you also have to disable the “Send invitations to attendees” in the Groupware settings of NC.
(Otherwise some people get two email invitations. One from the client and one from the server, but not in a reproducible manner as far as I found out. See also Proper way of sync with Thunderbird/Lightning AND working invitations)
I think a disadvantage of this solution is, that now no invitations are sent via email when you make an invitation using the web interface… If the invited person is on NC the event appears there but when he is not he might never get any knowledge of the event…