The problem has been fixed. Thank you!
There seems to be an issue when logging in to the League of Legends client. As a result, the client is temporarily being put under server maintanance as our team works hard to find a solution to the problem at hand.
We thank you for your patience and understanding on the matter. We sincerely apologize for the inconvenience cause and know that we are doing all we can to get the client up and running very soon.