Are you doing your first backup to rclone? If not, do you have an old backup to test that old AuthID?
If you followed good practice and safely preserved old Export To file, the AuthID is possibly in there.
AES Crypt or Duplicati’s included SharpAESCrypt.exe CLI command can decrypt the file if needed.
I suppose you could ask Dropbox support if they can set your account to old behavior for awhile…
Possible awkward workaround is to see if rclone can use Dropbox, then have Duplicati use rclone.
For rclone help, they also have a forum. I do see some varying posts about their Dropbox situation.
dropbox: support new oauth scheme #4792 looks like they at least tried to fix this before the cutoff.
Dropbox token expiring how to create non expiring token. is kind of unclear. Maybe title is off-target.
If you have other storage available, Duplicati destinations can be moved, then edit backup to use it.
Duplicati won’t do a location-to-location transfer, but there are other tools around that can be used.