Error Code C5 : an unexpected error has occurred during ready check

Hello everybody since a few Day's i have an Error what called C5 if i'm in a lobby with friends. When I'm alone in a Lobby and trying to play a Game this Error shows up: an unexpected error has occurred during ready check I dont know what to do i guess i did all what i can do by myself atm. ip release - renew, flush my dns, reapair and reinstalled League. anybody knows how to fix it ?