Did you disable power management and the screen saver? If Windows “turns off” the screen or the screen saver kicks in it will also result in a black screen shot.
If everything else looks good then you may have to disable hardware-accelerated graphics in IE. It should already be done by WPT but if you have a GPU in the test machine and IE uses it for hardware acceleration then the screen shots may not work. It is an option in IE’s advanced settings.
I have a private instance running on VM. I have disabled screensaver settings (changed the registry settings for screensaver) and enabled autologon. And followed the above mentioned instructions of restarting the instance rather than disconnecting RDP. I still get screenshot errors. Is there something that I’m missing ?