Dropbox API (59) Unknown Cipher Error

When I try to reauthenticate Dropbox I get an API (59) error. Any ideas what might be going on?

BackWPup version 3.1.2-beta3 (same problem on 3.1.1)

https://www.dropbox.com/s/0mnidbub3ilnixs/dropbox-error-backwpup.png

http://wordpress.org/plugins/backwpup/