Just fixed the Critical Error bug
For those of you that have been getting the "critical error" message upon completion of champion select, I just realized that there was a League client background process that was stuck running even after closing the game. All I had to do was force-close it and then re-open the league client. I have no idea if this will work for other people but it fixed me up right quick.
It took me 2 days to realize it was still open as a background process. I am not a smart man.
Edit: Realize it might not work for everybody but it is worth noting that this process somehow continued to run through multiple reboots and reinstalls