RSS per category stopped working

I was using the /wp-rss2.php?cat=3 to show the rss feed for category 3 and it was working just fine. Now I get these messages when I click on the rss link:
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WordPress database error: [Unknown column ‘category_nicename’ in ‘where clause’]
SELECT DISTINCT * FROM wp_posts LEFT JOIN wp_post2cat ON (wp_posts.ID = wp_post2cat.post_id) WHERE 1=1 AND 0=1 AND (category_id = 3 OR category_id = 4) AND post_date_gmt <= ‘2005-08-23 19:36:59’ AND (post_status = “publish” OR post_author = 1 AND post_status != ‘draft’ AND post_status != ‘static’) AND category_nicename = ” GROUP BY wp_posts.ID ORDER BY post_date DESC LIMIT 0, 10

WordPress database error: [Unknown column ‘category_nicename’ in ‘where clause’]
SELECT DISTINCT * FROM wp_posts LEFT JOIN wp_post2cat ON (wp_posts.ID = wp_post2cat.post_id) WHERE 1=1 AND (category_id = 3 OR category_id = 4) AND post_date_gmt <= ‘2005-08-23 19:36:59’ AND (post_status = “publish” OR post_author = 1 AND post_status != ‘draft’ AND post_status != ‘static’) AND category_nicename = ” GROUP BY wp_posts.ID ORDER BY not (ID in (18,10,19)), post_date DESC LIMIT 0, 10

Warning: Cannot modify header information – headers already sent by (output started at /home/xxxxx/public_html/xxxx/wp-includes/wp-db.php:98) in /home/xxxxx/public_html/xxxx/wp-rss2.php on line 8

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I’ve looked at the wp-db.php and the wp-rss2.php for spaces at the end of the files. That didn’t help.

Can anyone help?

Thanks