The only problem with this mission is that 1WP Enemy north can't start line abreast from the position you have them placed. There appears to be some terrain obstacle or something. Move them to the west about a half-click or so, or change the formation. The mission will then work without a hitch.
What occurs is that 1WP Enemy north spawns facing north, but moving backwards in the programmed direction of travel, which is southwest. About two seconds after the mission begins, the column tries to "respawn" facing the programmed direction of travel, but there seems to be a vehicle or two which is unable to change positions so instantly because of, as I said, some terrain obstacle or such. Hence the lockup.
I hope this observation is useful.