That's weird! It is the first time that some kind of problem with fog has been reported. In fact the fog should brighten your scene and not darken. Have you tried using only the fog effect on the camera to find out if there is any incompatibility with another image effect? Maybe it also could be an incompatibility with your system or target platform, but this hypothesis is not very valid, because you said the skybox is working, and the codes of skybox shader are practically the same as the fog. The only way for me to be able to correct a possible error is if I can reproduce the problem here.