I just wanted to check in here and report what worked for me, i just had the same problem.  (Im running LineageOS on a Fairphone 3, but I think this should be pretty transferable for android in general, lmk).  After seeing Firelord's answer (props, this is a great lead), I enabled developer options, debugging over usb and root debugging and connected my phone to my laptop w a usb data cable (confirm that you have a cable for data and not just power, this is important).

Install adb on your laptop.  You can then connect to your phone and reset your backup password with the following cli commands (you shouldnt need a rooted phone to do any of this):

`adb root`
`adb shell` 
(this will open up a shell session on your phone from your laptop)
`rm /data/backup/pw*`

Then reset your phone.  Your "desktop full backup" password should now be blank, and you can set up a new one.  Since youve already come this far, consider connecting w adb again and running 'adb backup' to download a backup to your laptop for future use - I personally find this to be a safer and more convienent option than juggling sd cards or ftp or whatever.