I am not an Android expert so I didn't try any root, flash, custom ROMs or custom recovery modes. All I did was approving to update my HTC One M8 to Android 6.0. After downloading updates and installation finish, my device booted into stock recovery mode with the message:
I've tried wiping cache and factory reset several times with no success.
P.S: I don't have SD card on my device and I tried booting with one SD card inserted and the same message show up.