Why does secondary screen turns black when selecting "Mirror Display"
I solved the problem by resetting NVRAM / PRAM:


Shut down your Mac.
Locate the following keys on the keyboard: Command (⌘), Option, P, and R. You will need to hold these keys down simultaneously in step 4.
Turn on the computer.
Press and hold the Command-Option-P-R keys before the gray screen
appears.
Hold the keys down until the computer restarts and you hear the startup sound for the second time.
Release the keys.