Client won't let me lock in champion pick

This has happened twice now. I will randomly have games where I can't lock in my champion pick. I click the champion that I want to play, and the Lock In button is greyed out. It doesn't matter if I pick a different champion, or what I do, it won't let me lock in. This last time, I was stuck in the lobby afterward, and saw everyone leave.

I have made no changes to my computer, nor have I done anything differently. I have switched to Google Fiber.

Now I have Leaverbusters because of this. This is ridiculous. There's no way I can reliably test what's happening, and make sure that I don't impact someone with a negative experience, since it happens 1/5 games.