I'm having exactly the same problem with the game I'm currently working on. 1.15.3 as well....
Try the following: At the end of level 2, save your game, and then completely close it (return to Windows). Then restart and resume your saved game - you will surely find level 3 loading properly.
It has something to do with decals, I guess. Because sometimes my level loads correctly and then crashes whenever I shoot an enemy (blood) or a flak weapon (smoke/explosion decal).
Anyway, my solution is, so far, I'm having kind of "checkpoint" levels which are empty between all "real" levels with an automatic Quick-Save-Script. So whenever a player will experience that problem with the game, he can easly jump back to where he was in-game without need to always remember saving.
This is at least a way to "go around" the problem, although it does not really fix it
... I hope TGC will have a find a way to fix that someday.