Hmmm... That's seems rather unusuall, o maybe familiar, idk.
Anyways, try maybe re-installing FH? Maybe It'll help?
If not, try checking you're practical folder, for any _0, _1 ending on or so... Maybe it will help, In any way? :/
Anyways, I wish you luck! ^.^
Sorry, for my bad explanation. :c
From the manual section here on the site explaining heightmaps
(~Note~)
If you were to create a heightmap on your own there's a few preferences it must follow and it has the very same as the heightmap used above and below. When your create/save a heightmap it must/should follow these rules.
1. It must be saved into a PNG format
2. It can only consist of 1 layer!(If you use more the game will crash upon importing it) Make sure you flatten/merge the layers on the image before saving it.
3. The image must be saved/edited in grayscale mode and cannot consist of or use colours(If not the game will crash)
4. The Image size should be even on both size, the standard size is 513 x 513 pixels.
5. The use of "Alpha Channel" should be left unmarked/turned off.
If something is not followed in that list it could very well crash your game until you sort it out.
Here it's explained in video.
http://www.youtube.com/watch?v=ZTO8tyhSQPU&list=PLCrTZGtLFJkJeB1qYGae-WGR_gSEj_xPf&index=1 (http://www.youtube.com/watch?v=ZTO8tyhSQPU&list=PLCrTZGtLFJkJeB1qYGae-WGR_gSEj_xPf&index=1)
Hopefully something there will help. ^^ You can also try changing your rendering system.
(http://i.imgur.com/MZWNkdn.png)
This is just after I flattened my image.
Flattening the image already makes the alpha channel go away, so you shouldn't have to worry about that.