hey, if you make a mask in the advanced output, it automatically blacks out your slice image a bit to put it to the background. If you then hit 'save and close' your slice ie. your whole output image(!) remains blacked out.
it is necessary to specifically deselect the mask item by either selecting the slice instead or clicking at the top of that menu. This is a recipe for distaster! No matter what is selected, the moment you hit save and close your slice image should jump back to 100% opacity.
this occurred on v4.2.1 on a mac.