Well, a workarround for this bug (caused probably for a bad update, at least in my case) is this:
1. Change the value of the db_version field in the wp_options table to 3307, like this:
UPDATE wp_options
SET option_value
= ‘3307’ WHERE option_id
=81 AND blog_id
=0 AND CONVERT( option_name
USING utf8 ) = ‘db_version’ LIMIT 1 ;
Then re-run the upgrade.php file and the comment count field of all posts will be recalculated.
Good luck!