I had the same thing with one of my elder simmies.
She and her husband moved to another house, and this problem appeared for her, husband was just fine.
Funny thing though that she still could use the garden toilet, one from
www.meshkiste.de(Site is closed right now, so go here for pictures
http://www.modthesims2.com/showthread.php?t=183364)
She was about to die soon, so i didn't bother.
Don't play that neighborhood now (was about to get BFBVFS)