Thankfully, David Chumps already has the solution for this. You can read his detail instruction for fixing the bugs over here. However, the first step which involved editing database using PHPMyAdmin is kinda tedious, so I tried to figure out the right SQL query to do the job. And here it goes:

UPDATE wp_term_taxonomy,wp_terms SET wp_term_taxonomy.description = wp_terms.name WHERE wp_term_taxonomy.term_id = wp_terms.term_id

This query will automatically fill the description field in wp_term_taxonomy with the right content from wp_terms table. It does save a LOT of time, especially if you have a LOT of categories and tags (I have over a hundred in one of my blog).

Note that in some condition, the content of wp_term table is also messed up, so you won’t be able to use the query above and you must modify wp_term_taxonomy manually like suggested by David. In my case, 20% of the affected blogs have the table messed up.

Hope it helps.