In the Audible Manager, click on Check For Updates, and install the Sansa / SanDisk digital players plugin, at least be sure it’s installed from the list. The “nuclear option” may be the hot ticket as a last resort: be sure to have backup copies of your music first.
On the player, format the device using Settings > System Settings > Format > Yes. Remember, this will erase all media from the device, so have backups first.
Lastly, reauthorize the Sansa using the Audible Manager, and transfer your audiobooks! The Fuze accepts Formats 4 and Enhanced only; the more compressed formats don’t work with the latest firmwares. Format 4 has a very clean sound, but my personal favorite choice is format 3, which doesn’t run on the Fuze, unfortunately. (It does on my little e280.)
Formatting the device forces a synchronization of the secure clock, part of the DRM system, which just might be your issue here.
Let me know how it goes!