Same problem today.
Resolved by flashing diags into fastboot
Download usb_diags image
here
At uboot type
bist fastboot
Use
https://github.com/TobiasWooldridge/Fastboot-Kindle
sudo ./fastboot flash diags PW3/pw3-diags.img
sudo ./fastboot setvar bootmode diags
sudo ./fastboot reboot
In diag mode put update.bin in usb folder and reboot.
Everything fine now.