error: cannot modify header information… HELP!

I originally posted this under another title, but it hasn’t received a reply yet, and I think I needed to change the topic title. 😉

Any time I publish a post or edit a published post, I get this message:

WordPress database error: [Unknown column ‘user_level’ in ‘where clause’]
SELECT ID FROM wp_users WHERE user_level > 0

Warning: Cannot modify header information – headers already sent by (output started at /home/lattin1/public_html/greatfamilygadgets/wp-includes/wp-db.php:121) in /home/lattin1/public_html/greatfamilygadgets/wp-includes/pluggable.php on line 275

It saves my changes, but I have to click the back button and then make sure the changes have been posted. What can I do to fix this? It’s doing it on at least two of my blogs that I’ve recently updated to 2.1.

Thanks for any help you can give me!!!!
Barb