A funny thing, I've copied DESERTMOUNT15.MAP to DESERT15.MAP, but it still crashes, this time complaining about missing DESERT16.MAP. Now this looks more like a nightly bug, not a mod bug, but I will keep it here, until you tell me to report it elsewhere.
Edit: now I see how it works --- when I removed DESERT12.MAP and higher from Ruleset/terrains_FMP.rul, the mission is generated fine. So probably missing files...