Steps to reproduce:
1. Open attached project "test5.zip"
2. Go to Edit -> Project Settings -> Player
3. Go to Settngs for PC, Mac & Linux Standalone
4. Make sure that "Color Space*" is set to linear and Graphics API for Windows is set to Direct3D11 in "Other Settings"
5. Build and run for Windows
6. Press "Space" to take screenshot
7. Screenshot is black

note:
in user's case screenshot is corrupted and only partially black
screenshot is captured normally in play mode(in Unity Editor)

Reproduced with: 5.3.6f1, 5.4.1p1, 5.5.0b3 on Windows(DX11)

Not reproducible with DX9, OpenGLCore