If you can't connect to one person in a room it doesn't let you in, similar to the EA games. I have this problem often, too (unless they just made a room or something). I'm not sure how to fix it though, it just seems like bad luck to me.
I think there is a setting that you cannot join someone in the middle of a match or something like that.....I get the same error. If you look at the room you are joining, look at the settings for it...the last option says something about it.