Was he an observer? As I often have an issue where I cant kick observers. Most of the time, I can un-ready, go observe and kick him. Sometimes even that wont work; but that is a rarety .

Starting with observers for me also gives a likely chance for my game to crash.

I noticed this other message on another thread for your reference :
https://forum.faforever.com/topic/6343/who-is-paralon-shutting-down-lobbies