It won't let me queue for some reason. Happened after I tried to stop queuing when steam stopped working. Wouldn't let me exit the queue so I turned the game off and then on again to fix it. Came back to this error message. Now I'm just unable to queue for a match.
Last time it happened to me I was able to queue again after 6 or so hours even though I changed nothing.
My current head-cannon is that this only happens if you stay in a game after it's been marked safe to leave because a leaver is detected.
Both times this has happened to me is immediately after playing a game out even with a leaver and I think the game doesn't know how to close games like that so it thinks I'm still there even though it ended.