FYI only:
In Project Settings, Engine-Rendering, Enable alpha channel support in post processing (experimental), if you set this to “Allow through tonemapper”, the export of render textures will be broken, giving only black exported images.
I enabled this ‘feature’ to get transparent backgrounds for scene renders (per Render with transparent background - Alpha Channel - in Unreal Engine - YouTube), which worked really well! A few days later, I started noticing that my exported PNGs (from Export Render Texture) were no longer valid image files, but just black. I thought perhaps I had done something dumb to the render texture or somewhere else in the pipeline, when I remembered this setting. Changing it back to ‘disabled’ fixed the problem.