WordPress MU

I want to exclude certain category (e.g for category_ID=47) from the loop. I have try 3 kind of code (with minus sign):
query_posts($query_string . "&cat=-47");
or
query_posts("cat=-47");
or
query_posts('cat=-47');
But all of them are not work to exclude.

Just for checking, if I try using code (without minus sign):
query_posts($query_string . "&cat=47");
or
query_posts("cat=47");
or
query_posts('cat=47');
All of them are work to show posts only from category_ID=47.

Why in my case query_posts does not recognize minus sign? Is this problem with my database or with my query.php/classes.php? Is this typical of WPMU that can not exclude posts in query_posts?

My server configuration:
WPMU 2.9.2
PHP 5.2.9
MySQL 5.0.90-community-log
Apache 2.2.11