An unknown DirectX error has occured and League of Legends cannot start.?

So I've been getting these error messages since a few days for no particular reason. Sometimes my League client works, sometimes it doesn't. I get this error when I start a game, and during the loading screen, the window becomes smaller, and the client closes with the error message mentioned above. My latest DirectX is 11, and I didn't mess with the DirectX ever since I reinstalled my PC (few weeks ago). What causes this error and why it's random? I'm really afraid to jump into a game because of this.
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