I had the same problem twice with a Terror Ship. No message, just a clean exit. I'll attach a save game, although I'm running a number of other mods which might make it hard for you to play this. (I'm pretty sure it's your mod that causes problems; a number of times it happened that a Terror Ship map loaded, but my Skyranger obliterated a part of the ship.)
EDIT: after some hours passed, I managed to start the mission. However, this seems to be a recurring problem with Terror Ships.
EDIT 2: It happened again, this time on a Medium ship and giving me ROUTES/ox_UFO3b.RMP not found or ROUTES/ox_UFO3h.RMP not found error.