Client crashing in Champion Loading screen
This seemed to start happening right after the mini-update in the middle of the 4.20 patch. After champion lock in, there's the usual window closes, the League icon shows on the screen, a blackout, and the champion splashes are supposed to show up along with loading progress.
Instead, the screen stays black, and eventually there's a notice that the client has crashed. Reconnecting and retrying doesn't fix the problem. You have to completely quit the application and rerun it.
Note: this happened on and off before, but it's much more frequent now. I have to reset and rerun every other game because of it.