I have seen several messages regarding Backup failure with error code 14. I have had the exact problem as described when trying to backup to my external drive. I was connecting my external drive to the computer's USB port using an extension cable (the USB ports are at the back of the machine and not readily accessible). After many failed attempts, I tried connecting the external drive to the computer's USB port directly, without the extension cable and the Backup ran successfully. I have a Pavilion All in One machine running Windows 11.
Another thought might be the USB 2 or 3 port but I do not know which port I am connected to, I assume USB 3 but not certain.
I hope this helps.
Michael.