Home wired with ceiling speakers, amp shutting off when one set is connected

So I just bought a new house that is wired with speakers in the ceiling in a few of the rooms and deck with separate volume controls in each room. Everything works fine except for the deck.

First, I tried changing the wires in the back of the receiver (Yamaha RX-V567) to plug those into a separate channel, no dice.

Next, I pulled the volume knob off of the wall, and saw that the input and output wires weren't connected. Weird. So I connected them, and as soon as I did, the receiver shut off. I powered it back on, and it shut off right away. Apparently the previous owners must have had this issue and just disconnected those wires so the rest of the system would work. Assholes.

I thought maybe the volume control knob was the culprit, so I switched it with one in a room that worked, nope. Still shut the amp off.

So it looks like if the deck speakers make a connection to the receiver, it shuts off. What can the issue be here? Bad wiring? Blown speaker? Just want to hear some opinions on what the possible causes are and things to check before I start making calls that'll cost me a couple hundred bucks.

I thought about pulling the speakers down and switching them with another room's speakers, but for the life of me I can't get the damn things down. The holes on the grille are too small for anything to fit into and I can't pry from the sides. Any suggestions there would be appreciated.

Thanks!