This is an ID that is already logged in.

Ok, Hime posted a response regarding this issue. Hopefully it'll be fixed soon. At least they acknowledge that it exists. See her post here:

http://forum.nexon.net/MapleStory/forums...39749.aspx

Dear Maplers,

Some of you may be experiencing an issue that gives you the error “This is an ID that is already logged in, or the server is under inspection.” when you try to sign into MapleStory. We are actively working on finding a permanent solution to this issue and in the meantime if you are experiencing this issue, please submit a ticket as soon as possible. We will get you back in game as soon as we can.

Please follow this method for submitting your ticket regarding this issue.
Game Selection: MapleStory
Category: Tech Support
Sub-Category: Login
Summary: Already Logged In Error

Thank you for your patience and see you in game soon!