This problem had made me very pissed off for many times. As long as in Expert mode, when the boss reached 20% HP, then it will pop me out to the log in screen and had a system message "Connection to the server has been lost due to abnormal client status. (0x00031)"
Can u imagine u had spent 1 hour plus in Drakos Expert mode and then that Paris left 20% HP, then this problem came. It also applied to the other Expert dungeons, 80% chance that this problem will occur.