Flyff? Wasn't that one of those really low-budget cookie-cutter korean MMOs from like.. early 2000s? Many of those ran some DRM anti-cheat software (nProtect or something?) that often installed deeper in your system than you would ever feel comfortable with. Chances are, that software doesn't like Windows 8, as it's very different under the hood. Also, with the age of the game (as well as its apparent low budget design), it's possible the game itself may lose compatibility with newer OS's if it was that poorly coded.
If the "nProtect" software does work, then maybe you need to set the game to run in compatibility mode for XP. This is an XP-era game, after all.