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You also need to make sure there is only one layer. If there are multiple you will have to flatten them.Additionally, there can be no spaces in your terrain name. If there are, you must replace the space with an _ (underscore).
Try right clicking on your single layer and selecting the option "Flatten Image". Most drawing software allows you to do so, though if it is not one of the right click options you may have to search the menus at the top for it.
May I ask what art program you're using the create the height map?
Hmmm...make sure to double check everything right before exporting.-513x513-Grayscale-No alpha channel-One, flattened layer-.png-No spaces-mapnameTerrain.png is the nameI know it seems repetitive but those are the only things that could be causing your terrain to crash.
Have you tried a different heightmap?