Champ select bugged for me.

I've been having this recurring problem today when in champ select. I've been playing normal's with draft pick. Whenever its my turn to ban a champ, I select the champ I want to ban and i get a pop-up that the champ I've selected has already been banned even though they haven't. The first time it happened today I managed to get it working before the timer ran out but just now it happened again and I kept getting the same pop-up when I clicked on any champion and I couldn't lock in my ban. I ended up getting a 5 minute ban from it so I don't want this to keep happening because I know the time bans get longer. anyone know why this is happening? I'm also playing on the legacy client because I get unplayable lag on the new client. Is it my computer or the client that's the problem?