What kind of image editing program do you use?
FH terrain maps, for some reason, only seem to work when they're edited with GIMP or Photoshop. So, if you're using Paint or something of the like, this could be the reason for the crash.
I'd try downloading one of these (I use GIMP), and save your heightmap with that program. This should fix your issue, more than likely.
But, if you already use one of these for your heightmaps, you may have some corrupted file in your game, and you may want to try an uninstall/reinstall.
I hope this helps, floof!