In this case, the customer reported that when he did a viewport capture, the capture area wasn’t in the right place. In this screenshot of his display properties, the red area is where the viewport capture happened:
There’s no way to set the position of the viewport capture window (it is all done automatically), so these kinds of problems are most likely a problem with the monitor setup configuration. The customer disabled his third monitor, and the capture viewport worked. So he then lined up all three monitors in a row, and then the viewport capture worked.