The reason you get the message when trying to access older threads or subscribes, is due to the forum version. When Vbulletin went from version 3 to version 4, if the database wasn't correctly upgraded to work with version 4, then you get left with the message that you have seen. It can be rectified, but only by someone who has direct access to the database that was originally part over version3 of the site and not version 4.

Everything is still in the original database, but just not accessable at this point.