Time Machine fails backup to a time capsule's attached hard disk
Time Machine tries to track a UID for the volume and not a name, but it could be confused.

Have you tried setting the backup destination to none and rebooting with no drives connected and no network connection.

At that point /Volumes should be clean and you would be safe to delete any non-local drives at that point. My guess is it's perhaps a second route to the network - do you have a wired connection and WiFi to the AirPort/Time Capsule?

diskutil list will also get you more information about drives than mount in case you want to post more details and can't get Time Machine happy with these steps.