If I understand correctly (disclaimer: I don't have a bluetooth keyboard or a bora-era backup so can't test) you just need to get rid of /var/lib/bluetooth/<your:bt:mac:address>. Just to be on the safe side, I would move it somewhere else instead.

So, get a root shell, run

and reboot the device (not sure if this is necessary, but just in case the old data is lingering around in some daemon's memory).

Note that this will probably lose all bluetooth settings and pairings so you'll have to recreate them by hand. Do not try to restore the old backup again, make a fresh one after you get the keyboard working.