Hi,
Anybody has experienced that black spots? In my scene, there are some objects with a Principle Volume, all inside a box with another Principle Volume. I thought that black spots were occurring because of object intersections, but some spots are visible with no object contact.
What graphics card are you using?
Problems like this can occur with some graphics cards and drivers. And try up/downgrading the driver - this could fix it.
Thanks Patrick. I have a nVidia GTX 1060. I´ve never had problems with graphic card, before…
It’s maybe related to the Transparency Bounces setting (In Rendersettings → Lights Paths → Max Bounces → Transparency). Try setting them a bit higher.
I thought it was that - but it seems to be a bug related to overlapping volumes (there already have been a few Bugs in the past related to that.) Maybe this one. Nothing fixes it except disabling the outer volume cube… maybe you could split it into multiple renderlayers.
Thank you, Stuntkoala. I´ll try to solve spliting the render.
Stuntkoala, I solved splitting the outer box from the clouds. Thanks for the advice. 
As a workaround for correcting the bad geometry of Displace modifier, you can add Remesh (Voxel) modifier to get better volume.
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YAFU! I used the Remesh in all clouds, and the problem was solved! Thank you, very much, man!