I've had unknown error or unexpected error, come up either during champion select or during ban champion.
It comes and goes but it is really annoying because you get either dodge timer or worse I've had also 20 mins bans because of repetitive errors.
This issue like I said has nothing to do with the client *at least for me* but rather with the lobby, because it persists through various patches, last patch I must admit it was more frequent.
I now got a 15min dodge timer again.
Please treat this issue as still "open and to be fixed", because I saw some other posts from 2 years ago with the same issue. It hadn't happened to me before, but now it started.
At least, try to make a filter between people exiting the game to dodge champ select and others who are getting errors...