I was in the bazaar and when i logged out the dungeon message apearred. I had a friend try to port, it said i was busy as if i was in a tower. I when to olde town and tried to log out, same message. Had friend try to port, same message. When to the shopping district, same thing happened. Didn't happen in the commons though. Just though i might let you guys know.
I have gotten that same msg as well, but only after you beat malistare and you get returned to the commons and then try to log off. Is that when it happened?
This is just a guess on my part, but it seems after your ported back to the commons you are still part of malistare's dungeon until hence the instance message when someone tries to port to you. Hopes this helps :)
