Hello kamilb880,
When I downgraded my Kindle, I also got the message 'Update Error. An invalid update file has been loaded on your device." I opened Downgrader and selected my device (KT4). I got the **** 5.12.X

**** screen and my Kindle was not downgraded.
I just tried again. Copied the same firmware and started Downgrader again. I still got the **** 5.12.x

**** screen, but my Kindle was downgraded!
Maybe you should just do the process twice. Copy the firmware to your Kindle (ignore the error message) and start Downgrader. Check if the Kindle has downgraded. If not, copy the firmware again and start Downgrader again. This is what worked for me!