They've begun banning IP adresses after at least 50 usernameassword logins. I personally don't have issues since I use VPN and my router has a dynamic IP adress, but you guys might have.
Even when you use LAC (Lol account checker), SLC checker, Riot will ban your IP on their auth servers (auth.riotgames.com), on all available regions.
I think this is a bit too harsh, as they will ban your IP if you spam their auth servers through the legit website :/
But this is their way of protecting other people's accounts.
Oh and also if your IP adress is banned DO NOT TRY TO LOGIN TO YOUR MAIN ACCOUNT, as they may, and most probably will, ban it!
Take care.