Open up Finder, on the left hand side, click Applications.
Find League of Legends, right click on it and click Show Package Contents
Then go to:
(alternatively, while in finder, press COMMAND SHIFT G and enter: /Applications/League of Legends/Contents/LOL/RADS/system)
DELETE the temp folder, and relaunch your client.
(I think this has been solved by Riot and you no longer need to do anything.)
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