quicktime black iphone screen camera el capitan 9.3.1

When I plugged my iPhone 6 (9.3.1) into my 2012 MBP running El Capitan, launched Quicktime, started Movie Recording, and selected the iPhone as the "camera", the iPhone screen went entirely black. Hard resets wouldn't fix the problem.

I was able to restore the iPhone using iTunes on the MBP, but only after I used the Erase function via iCloud Find My iPhone.

I then tried the same movie recording process and, again, the iPhone screen went entirely black. However, this time, I couldn't use iTunes on the MBP to restore because iTunes kept saying that Find My iPhone needed to be turned off, even though I deleted the device from my iCloud device list.

The iPhone receives calls and texts, it's just that you can't answer because the screen stays black when the call comes in.

Any ideas as to what to do next?