RSVP doesn't work (Caldav)

Trying to use eM Client for its greats features (I used to use Thunderbird) … nice but the RSVP doesn’t work in my case with caldav.
ok on my smart phone, ok with Thunderbird but not with eM Client.
I send and invite eM … receive the feedback from the attendees by email (small msg saying that the event can’t be found so no update …) but no update done to the event (RSVP). Checking with Thunderbird and smart phone and event updated …
I tried to find solution, seen lot of requests about that issue but not solution … So thanks for your support.