the volume controll should not be turned all the way up for zone 2. i have seen many people try this, leave the zone 2 volume all the way up and use the impedence matching volume controll to turn it down in the zone 2 room, with yamaha and denon receivers and it always has the same result, amplifier shuts down.

you should turn the zone 2 volume to a level where you will actually listen at and then use the impedence matching volume controll in the zone 2 room to go down from there. the denon definately has enough power to run any in ceiling speakers and too much power for most which is more likely the problem.

as for your other problem, i have never come across this, it may need to be serviced.