When the rss poster task is posting, the minimum character check is bypassed. So it's probably just that for some reason it couldn't get the content from the feed.

To do what you want (combine threads with the same title), I think you'd have to modify the rssposter.php script. To merge any existing threads I believe you'd need a php script as well (as opposed to an sql query).

Do you still have the backup of the database from when you went to upgrade the site? You may have to start over or hire someone to do the upgrade for you. You may run into problems with trying to fix the posts with a php script or some other code if there was a corruption during the upgrade in the database or something else happened along the way. Personally I would start over with the upgrade or hire someone who is familiar with doing these type of things.