This connection error is due to the loss of internet connection to the maplestory server. This error appears whenever the maplestory server is down for maintenance or server check.
Solution:
Check if the maplestory server is online! Go to the official maplestory website and check whether there is a patch update or a server check on the day.
If not, check your Internet connection. Sometimes, slow internet connection speed may cause you to lose the connection with Maplestory server and therefore you return this error on your screen and disconnect you from the game.
