If you are using K2 theme in your wordpress blog then upgrading to wordpress 2.2, K2 sidebar modules will not work. The new version of wordpress has in built widgets this is causing the problem.

There is a quick fix for this, you can override the wordpress widgets functionality by installing a small plugin from Darrell Schulte, you can download the plugin here. Else paste the below code in a text file and save it as a php file and upload it to your plugins folder.

<?php
/*
Plugin Name: Disable WordPress Widgets
Plugin URI: http://schulte.mn/
Description: This plugin overrides WordPress Widgets and will allow K2 Sidebar Modules to function.
Author: Darrell Schulte
Version: 1.0
Author URI: http://schulte.mn/
*/

remove_action('plugins_loaded', 'wp_maybe_load_widgets', 0 );
?>

Via: K2 Forums