Hello,
I just picked up two C200 M1 machines second hand, and have run into an issue. I tested both before I purchased them, and they both POST'd fine so I purchased them. When I got them home I powered up one of them up. It was doing fine but seemed to stall before network boot. So I connected a USB keyboard to it and rebooted. When I did key input was not working so I power down the server. When I powered it back up the fans came on full speed and kept going, and the video output was completely messed up. I pulled power cords and tried again several times with some outcome. Did I damage the main board by simply connecting a USB keyboard while it was powered up? The only thing I can think to try is pulling the CMOS for a few mins, then putting it back and try again.