If I did I would power down and unplug the connection that issued the protect warning. I'd connect up a speaker of the appropriate load for testing in my rack or where the Denon is and then power up and see if it came back.

Given the story my bet is a wire pair shorted out. Given the distances and how speakers can be a few ohms you might see the short on the Ohm meter but that would not tell you much else.
Bob