When I try to open League, it blocks me from using my computer

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I played one TFT match. When I tried to start another one I got an error. I thought it might be caused by Avast so I closed League client, shut down all Avast processes and services and tried to open league client again. After logging in, instead of showing me the client, it opened a second window, as if I was in game but the second window's screen was completely black, unresponsive and made me unable to switch to any other window, because it would automatically go to the foreground. I was unable to interact with anything other than Windows start menu. It also opened 2 error pop-ups which I was unable to see, because the black league window was always in foreground. However, after switching tabs with Windows Key + Tab, I was able to make a screenshot - it's in the link below
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