Hmm, I decided to do some more research into it:
1) It seems that ANY file named worlda-19.nw.png will cause it to crash. To test this, I grabbed one of the map part images that didn't make it crash and renamed it to worlda-19.nw.png. (Oops, sorry Ibonic, this is what you did; I misread your post).
2) Entering worlda-19.nw causes the client to crash.
3) Copying worlda-19.nw and renaming it (to testworld.nw), and entering the copied level does not cause the client to crash.
4) I thought that it might have something to do with newmainmap.txt, but it looks fine to me.
Is worlda-19 a symbol of the devil?
