The Thread/Forum Read Marking Type for the forum has been changed to make it more user friendly, and easier to follow through which threads you have read and haven't read.
You may have noticed that your current read system is messed up following this change, this was unfortunately unavoidable as advised by vBulletin. To fix this click
"Mark Forums Read" towards the bottom of the forum index.
This change can be discussed in this
forum thread.