Currently if you set a SceneCapture2D's "Capture Source" to "Final Color (LDR with PostProcess)", the result of the screen capture becomes just a black texture.
If you set the SF_Tonemapper show flag to false for the Scene Capture, it becomes useable again. But the Tonemapper show flag is not exposed in the editor, so this must be done in code either in USceneCaptureComponent's constructor by setting
ShowFlags.Tonemapper = 0;
or in the SceneCaptureDetails by exposing the flag in CustomizeDetails by adding it to the ShowFlagsToAllowForCaptures array like so
and then unchecking the flag in the details panel for the SceneCapture2D actor.
This appears to have started between approximately Oct 27th and October 28th, as it was working on the 27th, but then stopped working on the 28th.
1. Create a SceneCapture2D actor and let it render to a TextureRenderTarget2D.
2. Set Capture Source of the actor to Final Color (LDR)
3. Create a new Post Processing Material, use the TextureRenderTarget2D as Emissive Color and apply the material to the SceneCapture2D actor or add a Post Processing Volume which uses the material.
4. Now the render target turns black.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-4745 in the post.