There's 64-bit PNG files?? Let's see:
- The stb_image library used by SOIL does not support 64-bit PNG: http://nothings.org/stb_image.c
- SDL uses libpng, which should support everything.
- Make sure the texture is using GL_RGBA16.
I suggest trying to manually create a 64-bit texture, to make sure that your OpenGL implementation supports GL_RGBA16. Ex: Create a simple gradient of red, in memory, then load that as a texture. Then once you have that working, try loading the texture with SDL.