Since the OP conducted a series of ping tests already, it seems increasingly unlikely that they could have a serious connection issue which would cause this error.

This is not an isolated incident.. there are many users who are consistently disconnected from the game without a clear explanation. An investigation into this problem should be one of the top priorities for the technical team at Square Enix. However, the odd variable in this particular thread would probably be the ISP. Since I haven't met anyone else playing from Korea, I have no idea if others using Korea Telecom experience this problem.