I got this error as well, and the problem is: You have placed an areaportal where it's not supposed to be.
Make sure it's as big as the door, except for the thickness, which is supposed to be 2 grid units less than the door. And don't forget that the door has to be "a odd number thick"!
I guess that just confused you :p, check this (
http://richdiesal.jedioutcastmaps.com/tutorials/br201lsn5.html) out instead.