Today I connected my MacBook Pro 15" (Late 2016) to an Apple Thunderbolt Display, using a Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter. When I closed my MBp and tried to Skype, the camera of the Thunderbolt Display did respond—I only saw a black screen where I expected to see myself.
After Googleing a solution for this issue, I came across the following:
The issue stems from an issue with a piece of software called VDCAssistant, which has been part of Mac OS X for a few revisions. For some reason, it’s not activating the camera when needed.
Stephen's solution is to stop and restart this daemon with the following command:
sudo killall VDCAssistant
After running this command, the camera worked fine. 😎