Hello everyone,
Yesterday I was looking for this issue and found this forum. Just wanted to write here how I solved it.
In my case I had the same problem as you all. And what was causing the issue was an open software.
Capture One was controling the camera by tethering and didn't let it work as webcam or in any other way.
So, make sure you don't have any other photography software running (even in second plane).
Hope it can be usefull for you as well.(and apologize my english).