Liegt wahrscheinlich daran, dass du LoL im Dock angepinnt hast.
Bei mir läuft der Client gut (Mac OS X).
Merke:
I will not pin LoL on the Dock
I will not pin it on my sock
I will not pin the old LoL Launcher
Of this I could not be any stauncher
I will not pin the LoL Client
Heaven forbid I pin the game-game
that would just be lame-lame
If I wish to be compliant
For if I do my LoL will break
Because my shortcut will be a fake