The contour shader is crashing because the default value for the shader is set too low. The code base is undergoing some significant convulsions at the moment so I can't get a new version out for that problem in the short term, sorry.

If the relight command isn't properly shading things then it's most likely that there are wild points on your surface. Use Window>>Histogram to determine the min/max and distribution of altitudes on your surface - watch for unexpected outliers. Those can be removed by doing a height clip operation to your expected min/max values.
One other thing you can do for absolute shading is to use Texture>>Shader Setup and pick the Altitude tab from the property sheet. Check Absolute Coloring to get coloring ranging between your set values (default 0 and 2500 for min and max).

As for the saving problem, I would recommend saving temporary work as a .mdr file, which will effectively just save the floating-point altitude image from memory to disk. If you really want to use PNG as your file format of choice, then save your 16-bit PNG surface (make sure that PNG Surface is selected as the save type) and when you load it back in, again make sure that PNG Surface is selected as the file type. Your save file will be scaled to the full 16-bit range (0-65535) on save so absolute altitudes will be problematic.