Denon AVR-1907 blinking red power button
It sounds like you have a bad speaker on the zone 2, or you have the speakers connected improperly.
When you engage the zone 2 a relay clicks and the speakers then become part of the amplifiers circuit.
If the problem was in the amplifier, it would go into protect mode without even having speakers connected to it.
If the speakers all check good, you most likely have the wrong type of speakers, meaning the impeadance is too low, or you have too many speakers connected to it.
On the back of the Denon, you will see right around the speaker output jacks, what ratings the speakers should be. If I remember correctly, it says 6 ohms or more when using only "A" or "B", and it says 12ohms or more for "A" + "B".
If you have two 8 ohm speakers on the same speaker jack you now have a 4 ohm load. That is below the ratings and can cause the amp to shut down (go into protect). Since your amp shuts down right away, I think you have a bad speaker or way too many speakers connected.
I have seen many times when people will connect 2 or 4 speakers to the one of the outputs that are designed for only 1 speakers.
If you are using too many speakers you will need a speaker distribution box. It has a resistor network built into it so the amp never sees a lower impeadance than what it is designed for.
You can connect 4, 6 or 8 speakers to one set of outputs designed for 2 speakers. You do loose a lot of power to the speakers, but it is better than blowing up your amplifier.
If I can be of any further help, let me know.