I've been playing the game on Xbox, with DLC and stable for many months now. These last couple of months I can play one game online, then the next game as soon as the second player's turn starts EVERYONE in the match gets the "session no longer available" message and we ALL get thrown to the menu. We setup and play another and then, second game in second player's turn it happens again. The cycle continues.
These are in custom matches, it happens private & public and it happens no matter who the host is. It also happens regardless of who's playing: Separate teams of friends have independently experienced the same bug.
The only workaround is to leave the lobby between matches, but then you lose your players of course.
I would love to see a fix for this. Weirdly it has only been happening since the launch of Revolution, could it be a server issue at your end?