It's a bug -
http://tracker.vbulletin.com/browse/VBV-6125
It needs to be done via that database - the routenew table, field regex - and there should be two rows to change (the routeids should be consecutive). Actually, if it is a parent forum, then you would have to also change it for all the rows of the child forums also.
Make sure you do a database backup prior to making ANY changes to your database!!!!