[CLIENT] Black Login Screen Upon Launch 4.21

Bug: On every released version of 4.21 to date, I have been unable to successfully log in to League of Legends. Upon selecting "launch" in the launcher, the login window will appear but the space usually occupied by splash art and a log in option will be black, yet the borders and buttons of the window will be visible. I have re-installed the game several times, attempted to repair it, adjusted computer settings, verified my antivirus programs are working correctly and more, but I cannot seem to fix the issue. I've experienced this problem since patch 4.21 was first released.

Steps to reproduce:

1. Open League of legends.
2. Left click "LAUNCH" button.
3. Observe the black box within the log in window.

Here is an image of the login window.