[MattR]

My best guess is that because your device is UMS, that is why you are having trouble. My understanding is that UMS doesn't support the copy protection and DRM like MTP does, thus making it unlikely a subscription-based service like Audible would support it.

On the other hand, when I initially plug in my T10, I notice that there is a significant delay before the Audible Manager recognizes it. It's not instant, and during that time it shows the same thing that you describe. After a short while it does recognize it and then I can proceed with syncing.

Waiting long enough may or may not be your issue, but is a possibility. You don't mention any other issues related to other files. What about MP3s, OGG, etc...? Can you transfer those?

One other option to try would be to copy the Audible file directly to the \Service\AUDIBLE\ folder.