when I downgrade from adnoid 9 to 8.1,it happens .The phnoe shows"Your device is corrupt. It can't be truster and will not boot. Your device will shutdown in 30 seconds." And it's actually shutting down all the time without system load! =(
I can enter Download mode only. Platform tools also see my device . I couldn't access recovery mode. When i hold vol.+ and plug in USB, that warning shows again