I found a temporary fix for allowing league client to run for us affected (mostly windows 7 users) First launch Riot client and login to PBE, go to task manager and close LeagueClient.exe
Go to your Live League of Legends folder Usually located in C:/Riot Games/League of Legends/ Copy all files (not folders) except for system.yaml
Now navigate to C:/Riot Games/League of Legends (PBE)/ And paste all files replacing each one with the ones from the live folder above. Now press play button on riot client.
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