Ondra on every subject regarding this issue You give a "duplicate" link to another subject where You also put "duplicate" link to the same subject...
Actually I have the same problem and haven't found any solution. When using DR or backburner- all my render elements save black. It doesn't matter if I save them as png, tiff or openexr. I always get totally no information. It used to work before version 1.2, after updating to 1.2 everything was OK for few weeks and than Corona started to save all elements black. It happens only when I check the save file and add a path for files in the Common Tab. When the render finishes and I try to save the passes manually, everything is ok.
Unfortunately with backubrner You don't get the chance to save elements manually, so all the channel are lost...