there is some strange behaviour in new notification system in 17.1. I mean the side bar system. When add new event and add participants one by one from the list of users, then is everything all right. Each participant will receive notification and can react by one click on tentative,reject or accept in the notification side bar…
But when add group of participant, then each participant of that group have to click twice on tentative,reject or accept. When such user click just once nothing happened. So no response to event creator and no change in calendar. When such user react (click) in mail notification, everything goes right. One click is enough.
So the problem is only in side bar notification system. Can you verify that, the same is happening on your system ? Or point me how to fix it ?