I rezzed some voxels in an area of the sandbox and they shows as transparent with the star background showing through. I was using Nvidia GTX680 GPU on Windows 8.1 Pro.
@Judas nearby told me that he could see the voxels coloured normally. He believed that the building I was close to was using PNG textures, and that the issue might be the alpha layer priority issues seen on other systems.
I moved away so nothing was visible in my viewport and rezzed another voxel but that also showed as transparent.
But when I logged off and back on the voxels showed normally. Clearly the PNG textured building nearby had affected my view even when that building was not visible. A relog seemed to be necessary to clear the problem.