If all else fails an uninstall+reinstall might help.. And if that doesn't help re-download the client. But before you go through all that trouble do what the other suggested, change the compatibility. Maybe restart the computer.
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I mean it sounds like it wouldn't work but a lot of times when stuff doesn't work for me I just restart my computer and boom! it works. I mean seriously some of the simplest, maybe stupid, solutions help. Like when I was trying to install Ragnarok. It wouldn't install. Then a friend told me just to shake the window while it was installing/updating.. And it worked. But anyways.... Yeah..