The warren entrance in the kobold quest is bugged, you can't go through.
This bug was also listed here: http://www.escapefromunderdark.com/forums/showthread.php?t=19588
This is definitely mysterious, but I'm going to try a fix, and make a trigger transition as well in case that doesn't work.
(Johannes, for some reason obj_used_jump doesn't appear to work on its own in a QA)