Wrong posts count

WordPress suddenly (after update to 3.0?) displays a wrong number of posts for a particular category. There are no posts at all in this category (count zero) but instead WordPress displays a count of 24 both in the front and back end.

I don’t want the category to be listed at all in the sidebar on the front end, normally it does not when there are no posts in it, but now it is listed with 24 posts although there aren’t any. If I click on the category to display the 24 posts it (correctly) says that there are no posts available.

Thanks for any help.