It's probably not the same thing, but when mine went to update I initially got a "DEVICE ERROR" message. I couldn't turn off the Yamaha, either. I felt panicked but then I saw that the USB-powered WiFi LAN adapter I had plugged in was not receiving power. I used an external power supply for the wifi lan adapter, unplugged and plugged the Yamaha back in and it updated without issue at that point.
So if anyone is using a Wireless LAN adapter that is USB powered (I use the tiny awesome IOGEAR unit) be sure it's plugged in with the optional external adapter instead of USB through the Yamaha if you're doing a firmware update.