This means that there must be another portal within 1000 blocks of yours. It's a common problem when people have 2 portals close by. It generally causes people to end up going through the same portal in the nether. I suggest you find out the name of the other guy who closed up the house around the nether portal and ask him if you can have permission to go through (since you both share a nether portal). Ask him to give you permission on the door by doing /cmodify (your name) and clicking on the door. If it works out, you and him should be the only 2 people walking through those doors.
In the meantime, see if you can use someone else's portal if you can't reach the other guy.
