The link doesn’t appear to be working unfortunately? If it doesn’t work you could just paste the map data as a txt file (what it is at its’ base level) and I should be able to make that run in a new project.
So, if the black areas are refraction issues, perhaps you’re running out of bounces? Have you changed your bounce count settings at all? It could be that your materials are just refracting internally and never returning light from the other side correctly.
Okay, that is baffling then. It’s like light is refusing to transmit through correctly?
Wait a minute, have you done anything with r.pathtracing.approximatecaustics ?