I had this problem too after installing the Seasons EP. I know I have seen information about this out there somewhere but I've been searching and searching with no success. The only thing I am finding is that the Seasons patch was supposed to fix this problem. If I remember correctly that's what took care of it in my game. But it looks like you may already have this patch installed..?
In case you need it here is the link to the Seasons patch:
http://thesims2.ea.com/update/getpatch.php?appVersion=1.7&variation=cd