I had a locked bootloader and broken recovery, аfter a failed installation of the last firmware with a missing serial number on my PRO 5.
I recently tried this method with different versions of windows and different PCs.
And always got the same error in Multidownloader.
The phone reboots into fastboot and there is still a locked bootloader.
It was possible to solve the problem only in the service center, using the MRT Dongle.
Are there any suggestions of what the problem might be? (for future generations)
I couldn't even get the multidownloader to start. Could you share your pro5.conf file which firmwares have you tried in which configuration?
edit: sorry, just realized it's re-uploaded to mediafire.