Try different usb cable or different usb port on PC.
Also, does your back button working when you boot into rom?
Maybe updating flashtool might help...
Don't be confused with that "debugging off" message, because you only need it when you are in normal mode. There is no point /need/purpose for debugging when phone is in flash, fastboot or charging mode (nor it is possible). Flashtool only report lack of it, but since you don't need it, just ignore it)
Sent from my C6502 using xda app-developers app